A catalogue of ultracompact mass-transferring white dwarf binaries

Matthew J. Green; Jan van Roestel; Tin Long Sunny Wong

doi:10.1051/0004-6361/202554925

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A&A, 700 (2025) A107

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Issue		A&A Volume 700, August 2025


Article Number		A107
Number of page(s)		22
Section		Catalogs and data
DOI		https://doi.org/10.1051/0004-6361/202554925
Published online		07 August 2025

A&A, 700, A107 (2025)

A catalogue of ultracompact mass-transferring white dwarf binaries

Matthew J. Green¹^,2^,3^★, Jan van Roestel⁴^★ and Tin Long Sunny Wong⁵^★

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK 73019, USA
³ JILA, University of Colorado and National Institute of Standards and Technology, 440 UCB, Boulder, CO 80309-0440, USA
⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
⁵ Department of Physics, University of California, Santa Barbara, CA 93106, USA

^★ Corresponding authors: mjgreenastro@gmail.com; j.c.j.vanroestel@uva.nl; tinlongsunny@ucsb.edu

Received: 1 April 2025
Accepted: 18 May 2025

Abstract

We present an overview and catalogue of all known mass-transferring binary systems that have white dwarf accretors and orbital periods shorter than 70 minutes, including the AMCVn-type binary systems and a number of related objects. We provide an overview of the literature background and describe recent breakthroughs, open questions, and connections to other fields. This overview is intended to provide a starting point for researchers who are relatively new to the fields of ultracompact binaries or AMCVn-type binaries. We define the scope of the catalogue (the criteria for inclusion) and describe its format and contents (what data have been included for each target). We also summarise the observed properties of the known mass-transferring ultracompact binaries, including their orbital periods and component stellar masses and radii. Our aim in presenting the catalogue is to provide a central resource to enable researchers to track the current status of the known sample. At the time of writing, the catalogue includes 123 confirmed binaries and 29 candidates. The catalogue is hosted on a public platform (Zenodo) with version control, and will be updated regularly.

Key words: catalogs / binaries: close / stars: dwarf novae / novae, cataclysmic variables / white dwarfs

© The Authors 2025

Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model.

Open Access funding provided by Max Planck Society.

1 Introduction

Sixty-five years ago, Greenstein & Matthews (1957) took a spectrum of what they believed to be a peculiar helium-atmosphere white dwarf¹. The spectrum showed a series of broad helium absorption lines with double cores. Several years later, Smak (1967) observed a photometric signal with a period of 18 min. He suggested the interpretation that the object is an unusually compact binary system, with the photometric period corresponding to the orbit of the binary. The object was given a variable star name AM Canum Venaticorum (AMCVn)². Over the next two decades, variable objects GP Com (Warner 1972) and CR Boo (Nather 1985) were proposed to be binary systems of a similar nature.

Over time, our present understanding emerged: there exists a class of mass-transferring, ultracompact binary systems (mass-transferring UCBs), with orbital periods in the range 5–70 min. Each of these binary systems consists of a central white dwarf accreting material via Roche lobe overflow (RLOF) from a low-mass donor, which is either degenerate or semi-degenerate. The degenerate natures of the donor stars are necessitated by the orbital periods, which are shorter than can be achieved by mass-transferring binaries with non-compact donor stars (e.g. McAllister et al. 2019). They can be thought of as double-degenerate counterparts to cataclysmic variables (CVs; e.g. Warner 1995) or as mass-transferring counterparts to double white dwarf binaries (DWDs; e.g. Maoz et al. 2018; Munday et al. 2024).

The accreted material is dominated by helium and often enriched by metals such as nitrogen (e.g. Marsh et al. 1991). In most cases hydrogen is not detected at all. Several of the systems that are often termed AMCVn-type binaries, which we take here to mean systems with no detectable hydrogen, have upper limits of order 10⁻⁴ on the hydrogen fraction (Nagel et al. 2009; Green et al. 2019). However, there are a number of mass-transferring UCBs that do have detectable levels of hydrogen, as we discuss below.

For many years the number of known mass-transferring UCBs grew slowly: until the year 2000, fewer than ten mass-transferring UCBs were known. The dawn of large surveys has accelerated that growth. By the review of Solheim (2010), the number of known systems was 25, and that increased to 56 by the time of the population study by Ramsay et al. (2018). The present catalogue contains 123 confirmed systems and a further 29 candidates. Figure 1 shows the growth of the known population over time, and the influence of several large-area surveys.

For the first time, the known population is large enough to derive population-level statistics, a promising avenue for testing formation models. This will require targeted follow-up of the known systems, especially those from which the most information can be learned (such as bright or eclipsing systems). Given the accelerating growth of the known population, it is becoming harder for individual researchers to keep track of all known systems. We therefore present a catalogue of all known mass-transferring UCBs; we will update the contents at regular intervals. We hope that researchers will use this catalogue as a starting point for observing campaigns or as a basis for population-level statistical studies.

In the following section, we give a brief review of our current understanding of mass-transferring UCBs. The catalogue itself and an overview of the systems that it contains are described in the subsequent sections.

Throughout this work we use the term UCB to refer to any binary system with an orbital period <70 min; the terms AMCVn-type binary or classical AMCVn binary to refer specifically to any mass-transferring UCB with no hydrogen visible in its optical spectrum; and the term helium cataclysmic variable (He CV) to refer to any mass-transferring UCB that is significantly helium enriched, but still has clear hydrogen lines in its optical spectrum. The need to distinguish between the last two subclasses is driven by a number of observational differences, which we discuss in Section 2.4. The binary that was discovered first, and which gives the class its name, is referred to as ‘AMCVn itself’, to avoid confusion with the class name.

Fig. 1

Rise in the number of known mass-transferring UCBs since Smak (1967) first suggested that the variable star AMCVn may be a 17 min mass-transferring binary system. Also plotted are the times at which several large sky surveys went live, contributing to the rapid growth in the known sample since the turn of the century.

2 Background and overview

2.1 Accretion behaviour

Most mass-transferring UCBs have an accretion disc centred on the accreting white dwarf. The physics of a helium-dominated accretion disc has been explored by several works (e.g. Kotko et al. 2012; Cannizzo & Nelemans 2015; Coleman et al. 2018; Cannizzo & Ramsay 2019). The observed population can be broken into several categories by the behaviour of the accretion disc: whether it is in a permanently bright and ionised high state, a permanently cool and faint low state, or alternates between the two through a cycle of dwarf nova outbursts³. These categories are grouped relatively clearly by orbital period (Figure 2).

These trends can be explained by considering the large dynamic range of allowed mass transfer rates, and the steep dependence of mass transfer rate on orbital period. The dominant form of angular momentum loss is believed to be gravitational wave radiation, which depends strongly on orbital period: under the assumption of a zero-temperature white dwarf donor, it can be shown that the mass transfer rate depends on orbital period as $\dot{M} \propto P_{orb}^{- 14 / 3}$ $\[\dot{M} \propto P_{\text {orb }}^{-14 / 3}\]$ (for a more in-depth discussion, see Cannizzo & Nelemans 2015). At short periods (P_orb ≲ 20 min), the high $\dot{M}$ $\[\dot{M}\]$ leads to a hot, stable, and ionised accretion disc. As $\dot{M}$ $\[\dot{M}\]$ decreases with increasing P_orb, the disc encounters a thermal-viscous instability and exhibits dwarf nova outburst behaviour. Among systems that show dwarf nova outbursts, the outburst recurrence rates, durations, and amplitudes show strong dependences on orbital period (Levitan et al. 2015; Cannizzo & Nelemans 2015; Cannizzo & Ramsay 2019). At even longer orbital periods (P_orb ≳ 50–60 min), $\dot{M}$ $\[\dot{M}\]$ is too low to drive dwarf nova outbursts, and the disc exists in a persistent low state. The boundaries of the classes in period space are somewhat fuzzy, which may be evidence for a diversity of masses of the accretor stars, evidence for a range of levels of entropy in the donor stars, or evidence that instantaneous $\dot{M}$ $\[\dot{M}\]$ values can deviate somewhat from the expected secular $\dot{M}$ $\[\dot{M}\]$ .

During outbursts, a system brightens by 2–6 magnitudes for a duration of 10–100 days. Very often a periodic variation is seen during outburst, termed the ‘superhump’ period (e.g. Patterson et al. 1993; Kato & Osaki 2013). This originates from an interaction between the orbit and the precessing disc (the exact nature of the interaction is debated), and occurs at the beat frequency between the disc precession and orbital frequencies. As this is typically within a few per cent of the orbital period, it can be used as a proxy for systems for which the orbital period has not been measured directly.

The superhump frequency may provide some insight into the nature of the system. For unevolved, hydrogen-transferring CVs, an empirical relationship has been found between the superhump period, orbital period, and the mass ratio. Several forms of this relationship have been put forward (e.g. Patterson et al. 2005; Knigge 2006; Kato & Osaki 2013; McAllister et al. 2019). It is common practise to apply these same relations to helium-transferring UCBs in order to derive the mass ratio, but caution should be used: these relations are not necessarily well calibrated for discs with these compositions (e.g. Pearson 2007). For YZLMi the superhump-derived q agrees well with the eclipse-derived value (Copperwheat et al. 2011b), while for AMCVn itself, the spectroscopically measured q disagrees with the superhump-derived value (Roelofs et al. 2006b). Further investigation of systems that show both eclipses and outbursts will provide greater insight.

A separate class of brightening event is also observed among AMCVn-type binaries, sometimes termed long-duration outbursts or long-period outbursts (Rivera Sandoval et al. 2020, 2021, 2022; Wong et al. 2021). These brightening events have to date only been observed among long-period AMCVn-type binaries (P_orb ≳ 50 min). During these events the system brightens by 1–3 mags for a duration of 100–400 days. Although they are termed outbursts, the observed properties of these events are substantially different to the more typical dwarf nova outbursts: they are longer and fainter, and are characterised by colours that are redder than the quiescent colour of the binary (while dwarf nova outbursts are typically bluer). The binary ASASSN-21au has been observed to undergo both long-duration outbursts and dwarf nova outbursts (Rivera Sandoval et al. 2022). The physical nature of these long-duration outbursts is not yet well understood, but it has been suggested that a temporary enhancement of mass transfer may be necessary, as the duration of these events exceeds the viscous timescale of the accretion disc (Rivera Sandoval et al. 2020).

At the shortest orbital periods (typically P_orb ≲ 10 min), the binary is too compact for an accretion disc to form, and the accreted material impacts directly on the white dwarf. The prototypical examples of these systems are HM Cnc and V407 Vul (Motch et al. 1996; Israel et al. 1999; Ramsay et al. 2002). While several models were proposed to explain the nature of these systems (Motch & Haberl 1995; Cropper et al. 1998; Israel et al. 1999; Wu et al. 2002), the direct-impact UCB model was best able to explain the photometric and spectroscopic observations of the systems (Barros et al. 2007; Roelofs et al. 2010) and had become broadly accepted by the time of the review by Solheim (2010). The spin of the accretor can act as an orbital angular momentum sink removing angular momentum from the orbit, but the stability of these systems is made possible by a tidal torque between the accretor and donor stars (Marsh et al. 2004; Gokhale et al. 2007; Kremer et al. 2015). The orbital period boundary between UCBs with and without accretion discs is not strict: Chakraborty et al. (2024) found two disc systems with P_orb ≈ 8 min, making them more compact than the disc-less V407 Vul (P_orb = 9.5 min), while a direct impact system has even been claimed with a period of 23 min by Ramsay et al. (2018).

Fig. 2

Period distribution of all known mass-transferring UCBs, colour-coded according to their accretion disc behaviour in the case of AMCVn-type systems (those without detected hydrogen). The He CVs (systems with detected hydrogen) and other classes are coloured separately. The accretion disc behaviour clusters by orbital period, but with some regions of overlap between the different subclasses. He CVs are typically found at periods ≳50 min. We note that most He CVs do show outbursts, while systems in the same period range without hydrogen typically do not. The orbital period distribution is strongly influenced by selection effects, most notably the fact that it is easier to find outbursting systems.

2.2 Formation scenarios

One of the prominent open questions in the study of mass-transferring UCBs is the question of their formation. Proposed formation models have generally coalesced into three channels: the white dwarf donor channel, the helium star channel, and the evolved CV channel. A schematic version of these evolutionary channels is presented in Figure 3.

In the white dwarf donor channel (Paczyński 1967; Deloye et al. 2007; Wong & Bildsten 2021; Chen et al. 2022), the binary system passes through two phases of mass transfer as both stars evolve off the main sequence, leaving two white dwarfs, one of which has a low mass. Following a period of inspiral, mass transfer from the low-mass white dwarf starts at an orbital period of ≈2–10 min (Wong & Bildsten 2021). If mass transfer is stable, the period evolution turns around after a short time to evolve outwards, moving quickly through the short orbital period range and more slowly through longer orbital periods. If mass transfer is unstable, then the binary will merge to form a high-mass white dwarf, hot subdwarf, or R CrB star; or may form a Type Ia supernova (Section 2.3). The peak mass transfer rate (shortly after the onset of mass transfer) is high enough in some systems to trigger helium nova eruptions on the surface of the accretor, which have the potential to destabilise the binary system (Shen 2015) or cause a thermonuclear detonation of the accretor (Wong & Bildsten 2023).

The helium star donor channel (Savonije et al. 1986; Iben & Tutukov 1987; Yungelson 2008; Bauer & Kupfer 2021; Sarkar et al. 2023b; Rajamuthukumar et al. 2024) follows a similar path, with the main distinguishing feature being that the donor star is burning helium in its core at the onset of mass transfer. This results in a more inflated donor star and pulls the onset of mass transfer towards slightly longer orbital periods of ≈10–15 min. Both helium novae and Type Ia supernovae are also possible for systems with helium star donors (e.g. Bauer et al. 2017; Rajamuthukumar et al. 2024). It may be possible to distinguish donors formed by this channel from others by their chemical composition, in particular by the ratio of nitrogen to carbon (e.g. Yungelson 2008; Nelemans et al. 2010). Non-mass-transferring WD+sdB binaries with short periods (e.g. Geier et al. 2013; Lin et al. 2024) may be progenitors to mass-transferring binaries formed through this channel.

The evolved CV channel (Tutukov et al. 1985; Podsiadlowski et al. 2003; Goliasch & Nelson 2015; Sarkar et al. 2023a; Belloni & Schreiber 2023) is a rather different process: the binary system passes through only one period of common envelope evolution, forming the central white dwarf, and begins stable mass transfer with a donor that is somewhat evolved off the main sequence. The outer layers of the donor are then stripped by this stable mass transfer, revealing the helium core, and the accreted material becomes helium dominated. Historically, models of this channel struggled to reproduce several features of the system, requiring significant fine-tuning to create binaries with periods ≲30 min or without detectable levels of hydrogen (e.g. Goliasch & Nelson 2015). Recent work has shown that the problems of historic models can be resolved by adopting magnetic braking prescriptions with higher rates of angular momentum loss. The Convection And Rotation Boosted (CARB) magnetic braking prescription, originally developed for X-ray binary evolution (Van & Ivanova 2019), also successfully produces AMCVn-type binary systems when applied to cataclysmic variables with evolved donors (Belloni & Schreiber 2023). Another possibility is the double-dynamo magnetic braking prescription, which increases the strength of the magnetic field on the donor star, and can also produce UCBs (Sarkar et al. 2023a). However, it should be noted that these enhanced models of magnetic braking, in particular the CARB model, do not well describe populations of single stars, or binary systems consisting of two main sequence stars (e.g. El-Badry et al. 2022).

All of these channels may contribute some fraction to the total UCB population, but the fraction of systems forming from each channel remains an area of active research. Each channel can recreate the parameters of most individual systems, and it is likely that the best test of formation models will come from a comparison of population statistics.

The eventual fate of the mass-transferring UCBs is somewhat uncertain. In principle, evolutionary models suggest that mass-transferring UCBs can evolve to P_orb > 70 min within a Hubble time (e.g. Wong & Bildsten 2021). However, the survey for eclipsing systems by van Roestel et al. (2022) did not find helium-transferring binaries with periods longer than 70 min, even though such systems would have been within the survey’s sensitivity. If mass transfer were to cease at a period of ≈70 min and if the donor were to survive, the next stage of the binary would appear like a white dwarf–brown dwarf binary system in which the white dwarf has a helium atmosphere. This has also been predicted in the context of ultracompact X-ray binaries (see Section 2.5) by Sengar et al. (2017). However, although several white dwarf–brown dwarf binaries were found at similar periods in the survey presented by van Roestel et al. (2022, 2025), none contains a white dwarf with a helium atmosphere.

Fig. 3

Schematic diagram of the proposed evolutionary paths by which mass-transferring UCBs can form, beginning with a solar-type binary system and passing through three proposed formation channels. The labels highlight the white dwarf donor channel (A), helium star donor channel (B), and evolved cataclysmic variable channel (C). See text for details of the channels.

2.3 Gravitational waves and type la supernovae

Besides the evolutionary questions discussed above, studies of mass-transferring UCBs are often motivated by either their strong emission of gravitational waves or by their evolutionary connections to Type Ia supernovae.

UCBs (both mass-transferring and non-mass-transferring) are loud sources of gravitational wave radiation in the frequency band that will be visible to space-based interferometers such as the Laser Interferometer Space Antenna (LISA; Amaro-Seoane et al. 2017) and TianQin (Luo et al. 2016; Ye et al. 2019). The shortest-period UCBs are expected to be among the loudest Galactic sources visible to LISA (Kremer et al. 2017; Breivik et al. 2018), and they are planned to be used as calibration and verification sources (Kupfer et al. 2024). Approximately half of the proposed calibration sources are mass-transferring UCBs.

Longer-period UCBs will contribute to an unresolved gravitational wave foreground which sets the detection limit of LISA at certain frequency ranges. In general it is assumed that non-mass-transferring DWDs, which make up the majority of UCBs by space density, will be the dominant source of this unresolved foreground (e.g. Korol et al. 2017, 2022). While the contribution of mass-transferring systems is likely to be weaker, its exact strength is difficult to quantitatively predict given current uncertainties around the population and its evolution.

Double-degenerate binary systems have long been thought of as candidate progenitors of type Ia supernovae, and in the last decade this scenario has gained in popularity. Reviews of Type Ia supernova formation and mechanisms are given by Maoz et al. (2014) and Patat & Hallakoun (2018), but see also work on the dynamically driven double-degenerate double-detonation scenario by Shen et al. (2018a,b). In general, a Type Ia supernova can arise from a double-degenerate binary if the two white dwarfs merge to form a super-Chandrasekhar mass white dwarf, or if a helium detonation on the surface of the accreting white dwarf causes a pressure wave that triggers detonation at its core (the double-detonation scenario). The double-detonation scenario can occur either during dynamically driven accretion just before a merger (e.g. Guillochon et al. 2010; Shen 2015; Shen et al. 2018a) or during stable mass transfer (in which case the surface detonation arises during a strong helium flash, e.g. Bildsten et al. 2007; Wong & Bildsten 2023), but in both cases it is crucial that the accreted material be dominated by helium. It should be noted that Piersanti et al. (2014, 2015) have argued that the stable mass transfer case of double-detonation should occur only rarely. In the double-detonation scenario, the accretor typically needs to be ≈1.0 M_⊙ in order to produce a typical-luminosity supernova. Masses of ≳1.1 M_⊙ can produce over-luminous supernovae (provided the core is still carbon-oxygen), while masses of ≈0.8–0.9 M_⊙ may produce subluminous supernovae (e.g. Shen et al. 2018b, 2021).

A minority of systems in this catalogue may themselves be Type Ia supernova progenitors. In particular, ATLAS J1138–5139 (Chickles et al. 2024; Kosakowski et al. 2024) hosts a 1.0 M_⊙-mass primary star and is believed to still be moving towards shorter periods and higher mass transfer rates. This system may be a Type Ia supernova progenitor in the next few million years if the mass transfer rate becomes sufficiently high. Two systems with subdwarf donors both have relatively low-mass accretors and may experience subluminous supernovae (Bauer & Kupfer 2021)⁴. A greater number of candidate Type Ia supernova progenitors have been identified among non-mass-transferring DWDs (e.g. Munday et al. 2024, 2025).

The majority of the mass-transferring UCBs in this catalogue can be understood as systems in which a Type Ia supernova did not and will not occur. The mass transfer rates in most systems with P_orb ≳ 10 min are too low to trigger a helium nova eruption. Additionally, the accretors in most systems with mass measurements are typically ≈0.8 M_⊙, which is too low to produce a typical-luminosity Type Ia supernova. Studying these systems can still provide some insight into the supernova question: by constraining the initial parameter space of the systems that exist today, we can demonstrate regions of parameter space in which a DWD does not host a supernova.

Fig. 4

Period distributions of the overall population of CVs (Ritter & Kolb 2003, seventh edition, updated in 2014), CVs with evolved donors (provided by B. Gänsicke, priv. commun. 2025), and AMCVn-type binaries (systems without detectable hydrogen, this work). Magnetic CVs and CVs with orbital periods below the non-degenerate period minimum were excluded from the CV period distribution. The orbital periods of evolved CVs cover a broad range; the shortest are ≲70 min, and therefore fall into the period range of this catalogue.

2.4 The diversity of mass-transferring UCBs

The descriptions given so far apply mostly to what we might term a classical AMCVn-type system. Systems of this subclass make up the majority of accreting white dwarfs with P_orb ≲ 70 min, and have a number of observational properties in common with each other. In addition to the absence of spectroscopic hydrogen lines (which we take as the definition of this subclass), they follow the pattern of accretion disc behaviour with orbital period described in Section 2.1, and they form a sequence in terms of their observed donor masses and radii (see Section 5.5). However, in the last few decades it has become clear that an impressive diversity of binary systems exist in this period range, including many systems which do not follow these patterns. This diversity is primarily driven by differences among the donor stars of the binary systems.

Between the classical AMCVn binaries and non-evolved CVs are systems that are variously known as He CVs, hybrid systems, or transitional systems: they have orbital periods of 50–70 min and contain detectable levels of hydrogen, while still being significantly helium-enriched when compared to typical CVs. The first-discovered of these was V485 Cen (Augusteijn et al. 1993), and around 20 of these systems are known today.

These He CVs are the short-period end of a broad distribution of CVs with evolved donors, which span a wider range of periods than CVs with unevolved donors (Figure 4). They are naturally connected to the evolved CV formation channel discussed in Section 2.2, which supposes that these systems are evolving inwards towards a period bounce, and that at least some fraction will then evolve into the completely hydrogen-depleted AMCVn binaries. The He CV with the most precise measurements of its component stellar masses and radii is the eclipsing binary ZTF J1813+4251, for which MESA evolutionary models do suggest that it will evolve into an AMCVn-type system after passing through a period minimum of 20 min (Burdge et al. 2022).

Two binary systems have been discovered in which the donor is a hot subdwarf (Kupfer et al. 2020a,b). These binary systems may also evolve into AMCVn binary systems, or they may be potential progenitors of Type Ia supernovae (Bauer & Kupfer 2021; Rajamuthukumar et al. 2024).

Two further unusual systems each consist of a warm donor with some detectable hydrogen in a 14 min and 28 min orbit, respectively, around a white dwarf (Burdge et al. 2023; Chickles et al. 2024; Kosakowski et al. 2024). The white dwarf donor channel is the favoured formation scenario for both of these systems, and they may either evolve into AMCVn-type systems or detonate as Type Ia supernovae.

Lastly, the catalogue also includes two binary systems with brown dwarf donors. Both binaries have orbital periods of 66–70 min, well above the theoretical lower limit of 40 min predicted by Nelson et al. (2018) for mass-transferring brown dwarf donors. For the first-discovered of these, SDSS J1507+5230, there has been some debate whether the donor descended directly from a brown dwarf (Littlefair et al. 2007) or from a metal-poor halo star in which the low metallicity causes the donor to deviate from a typical CV mass-radius track (Szkody et al. 2005). Measurements of composition and space velocity support the latter scenario (Uthas et al. 2011). In both scenarios, the donor today could be classed as a brown dwarf. The recently discovered WD J1540–3929 also has a brown dwarf donor and may have a similar history (Steen et al. 2024).

There is a remarkable absence of magnetic fields among UCBs. Among the whole population, only two binaries have been shown to host magnetic fields (Maccarone et al. 2024). The proposed field strengths in both cases are ≲100 kG, and both show clear observational evidence of an accretion disc. For comparison, some 30 per cent of hydrogen-dominated CVs in a volume-limited sample (Pala et al. 2020) are either polars or intermediate polars, with magnetic fields of order 10–100 s MG, which in the case of polars is strong enough to completely disrupt the accretion disc. The absence of comparable magnetic fields among UCBs may be related to the stability: the stability of accretion during the direct impact phase (P_orb ≲ 10 min) relies on a balance of torques between the accretor and donor (Marsh et al. 2004), and a strong magnetic field may disrupt this balance. The origin of magnetic fields in CVs is itself an area of active research (e.g. Schreiber et al. 2021), and the inter-relation between magnetic field formation and UCB formation is an aspect that should be considered by researchers in both areas.

2.5 Ultracompact X-ray binaries

This catalogue includes only binary systems in which the accreting object is a white dwarf, but another class of mass-transferring UCB exists: ultracompact X-ray binaries (UCXBs). Armas Padilla et al. (2023) give a recent review and a catalogue⁵ of known UCXBs. In these binaries the accreting object is a neutron star⁶, while the donor is a white dwarf or other (semi-)degenerate object. They typically have orbital periods ≲80 min, but can have periods as long as 120 min. The space density of these objects is much lower than that of white dwarf-containing UCBs, but their strong X-ray emission allows them to be identified at greater distances (2–20 kpc). The catalogue of Armas Padilla et al. (2023) contains 49 systems, of which 20 are confirmed, 25 are candidates, and 4 are related, short-period X-ray binaries.

A number of similarities can be drawn between UCXBs and UCBs containing white dwarfs. There are three proposed formation channels for UCXBs (e.g. Yungelson et al. 2002; Podsiadlowski et al. 2002; Nelson & Rappaport 2003; van Haaften et al. 2012), each of which is analaguous to one of the three formation channels proposed for AMCVn-type systems (Section 2.2). The donors in UCXBs may therefore be similar to the donors of AMCVn-type binaries, but a carbon-oxygen core donor star is also possible and has been proposed for some systems (e.g. Nelemans & Jonker 2010).

UCXB formation is significantly enhanced in globular clusters due to dynamical interactions, and 11 of the 49 known UCXBs are members of globular clusters. Those systems in clusters also have notably shorter orbital periods than the field population. Two candidate AMCVn-type binaries have been found in globular clusters (Zurek et al. 2016; Rivera Sandoval et al. 2018), but the sample numbers are too small for a direct comparison at present.

2.6 Large-area surveys and selection effects

The sample of known, mass-transferring UCBs is substantially inhomogeneous: systems in the catalogue have been discovered through a variety of means, in some cases serendipitously and in other cases as a result of targeted searches. The rapid increase in discovery rate over the last few decades is largely due to targeted searches using data from large-area surveys. We mark the first-light dates of several of these surveys in Figure 1 to demonstrate this point. As a result of this history, the known sample of UCBs is subject to a variety of complicated selection effects, which should be considered by any study that utilises the whole sample.

The Sloan Digital Sky Survey (SDSS; York et al. 2000; Abazajian et al. 2009; Eisenstein et al. 2011) has published both spectroscopy and (for a larger number of targets) five-colour photometry. A number of targets have been directly discovered through their SDSS spectra (e.g. Anderson et al. 2005; Roelofs et al. 2005; Inight et al. 2023a,b). Additionally, Roelofs et al. (2007, 2009), Rau et al. (2010), and Carter et al. (2013a,b, 2014a,b) implemented a spectroscopic follow-up campaign of candidate mass-transferring UCBs that were selected on the basis of their SDSS colours. That survey was believed to be complete (within the SDSS footprint and their colour selection criteria) to a magnitude of g < 19, and 70 per cent complete to g < 20.5.

Many mass-transferring UCBs have been discovered due to their dwarf nova outbursts (Section 2.1). Time-domain photometric surveys are ideal for this, such as the Palomar Transient Factory (PTF; Rau et al. 2009), the Catalina Real-time Transient Survey (CRTS; Drake et al. 2009), the All-Sky Automated Search for SuperNovae (ASAS-SN; Shappee et al. 2014), the Zwicky Transient Facility (ZTF; Bellm et al. 2019), and Gaia Science Alerts (Hodgkin et al. 2021). Outbursting UCBs have been discovered using data from all of these surveys (e.g. Levitan et al. 2011, 2013, 2014; Breedt et al. 2012, 2014; Campbell et al. 2015; Kato et al. 2015; Kato & Kojiguchi 2021; van Roestel et al. 2021). In particular, van Roestel et al. (2021) implemented a selection based on the colour of the target during outburst which resulted in a relatively high UCB true-positive rate of 25 per cent. Kato & Kojiguchi (2021) put forward a method to find UCB candidates from among outbursting binaries on the basis of their outburst lightcurves, which has also produced a large number of candidates. While the detection of outbursting systems is perhaps the easiest method to find mass-transferring UCBs, it introduces a complicated set of selection effects into the known sample. Naturally, only systems that show outbursts can be found in this manner (among the classical AMCVn-type systems this means orbital periods of 20–50 min, Figure 2), and between outbursting systems there is a further bias in favour of systems which outburst more frequently (the shorter period end of the outbursting systems). On the other hand, longer-period outbursters tend to brighten by a greater degree during outburst (Levitan et al. 2015), and so can be detected at greater distances. Overall, these selection effects are difficult to quantify and model.

The high time resolution of ZTF has enabled direct searches for eclipsing UCBs, dramatically increasing their representation among the known sample (Burdge et al. 2020, 2022, 2023; van Roestel et al. 2022; Khalil et al. 2024). An advantage to this approach is that eclipse probability depends only weakly on orbital period for mass-transferring UCBs, so this approach does not introduce a strong period selection except for the magnitude limit applied.

Lastly, mass-transferring UCBs are X-ray sources, and have been discovered through X-ray data from the Röntgensatellit (ROSAT; Truemper 1982) and the extended RÖntgen Survey with an Imaging Telescope Array (eROSITA; Predehl et al. 2021). This approach is particularly sensitive to UCBs in a direct-impact accretion configuration, which are the strongest X-ray sources (Motch et al. 1996; Israel et al. 1999; Haberl et al. 2017; Maitra et al. 2024), but has also yielded UCBs with accretion discs (e.g. Rodriguez et al. 2023; Schwope et al. 2024a). Schwope et al. (2024b) and Rodriguez (2024) gave overviews of the X-ray properties of mass-transferring UCBs in relation to other X-ray sources.

3 Criteria for inclusion

3.1 General inclusion criteria

Several overlapping sets of definitions exist in the literature for the terms UCB and AMCVn-type binary. These definitions may include a combination of short orbital periods, the nature of the component stars, the composition of the accreted material, and/or the evolutionary history of the binary system.

We adopt the following set of criteria for inclusion in this catalogue:

The target is a binary system undergoing RLOF.
The accreting object is a white dwarf.
The orbital period of the binary is shorter than 70 min, or there is reasonable evidence to believe this is the case.

Targets that meet these criteria are referred to as mass-transferring UCBs throughout this paper. These criteria were chosen to maximise the inclusivity of the catalogue while focusing on properties that can be confirmed by observation relatively cheaply. The existence of mass transfer can be confirmed by the detection of dwarf nova outbursts, spectroscopically visible disc emission lines, accretion-driven photometric flickering, or (in eclipsing systems) a photometrically detected bright spot. The nature of the accreting object as a white dwarf is clear from basic observational properties: mass-transferring binaries such as X-ray binaries appear quite different.

The orbital period (or the superhump period, which serves as a proxy for the orbital period) can often be measured photometrically, especially during an outburst, meaning there is no requirement for more expensive spectroscopic follow-up. The orbital period cut-off of 70 min was chosen so as to include all AMCVn-type binary systems (of which the longest orbital period is 68 min; Green et al. 2020), while avoiding cataclysmic variables with non-degenerate donors (which have a typical period minimum of 80 min, with scatter down to 76 min; e.g. McAllister et al. 2019). As AM CVn-type systems have a characteristic spectrum (lacking hydrogen lines) and all are within the period cut chosen here, an AMCVn spectroscopic classification is sufficient for inclusion in this catalogue even without a measured orbital period. The decision process by which targets are included or excluded in the catalogue is illustrated in Figure 5.

Many discoveries of UCBs are never published in a peer-reviewed journal – early observations are often carried out by amateur astronomers, and discoveries are often only posted to unreviewed forums such as the Variable Stars Network (VSNET)⁷ or Astronomer’s Telegram (ATel)⁸. Discoveries may be officially published in peer-reviewed venues only with a significant delay, or never. Therefore we do include such unreviewed forums as confirmation of the nature of the binary system, if there is (for instance) a clearly reported detection of an outburst with a superhump period <70 min.

Alongside confirmed systems that meet all of the above criteria, we also include a number of candidate systems. The inclusion or exclusion of the candidates is necessarily somewhat more ad hoc. Candidates are included when the above criteria have not been confirmed, but there is reason to think they are likely. Dwarf novae that are suggested to be UCBs due to their outburst shape and duration (the method proposed by Kato & Kojiguchi 2021) are classed as candidates in this catalogue, unless a spectrum or confirmation of the orbital period has been obtained.

Fig. 5

Flow chart demonstrating the process by which targets are included in or excluded from the catalogue.

3.2 Subclass definitions

We classify the majority of targets in the catalogue into one of several subclasses of mass-transferring UCBs. Here we give the definitions of those subclasses and the criteria used to identify them. These subclasses are discussed in more detail in Section 2.4.

AM CVn-type binaries: we take this to refer to mass-transferring UCBs with no detected hydrogen in their optical spectra. This includes the majority of confirmed targets in this catalogue (96 out of 123).

Several direct-impact systems (HM Cnc and the candidate direct-impact system 3XMM J0510–6703) do show hydrogen lines. However, evolutionary theories describing these systems position them as direct progenitors to AMCVn-type binaries that are passing through a short-lived phase in which the thin hydrogen shell of the donor is accreted (D’Antona et al. 2006; Kaplan et al. 2012). HM Cnc also appears to follow the overall trend of AMCVn-type donor star masses and radii (Section 5.5). Therefore, we consider the direct-impact binaries to belong to the AMCVn-type subclass, despite the presence of hydrogen.

Systems without observed spectra were classed as AMCVn-type if they have recorded dwarf nova outbursts and have P_orb < 40 min, on the grounds that every UCB exhibiting dwarf nova outbursts in this period range that has been followed up spectroscopically has been found to be an AMCVn-type binary.

He CVs: we use this term to refer to mass-transferring UCBs that show the presence of hydrogen but still have an enhanced helium abundance relative to the general CV population, and have P_orb < 70 min. These systems are generally found with P_orb ≳ 50 min and, as discussed in Section 5.5, appear substantially different to the AMCVn-type systems in terms of their donor masses and radii.

If no spectrum is available, we classify systems as He CVs if they have P_orb of 50–70 min and undergo dwarf nova outbursts with a recurrence time ≲3 years. The dwarf nova recurrence times of AMCVn-type binaries with such long periods are expected to be decades, if outbursts occur at all (Levitan et al. 2015). He CVs make up the second largest subset of confirmed UCBs in this catalogue (15 out of 123).

Two systems, CRTS J0808+3550 and CRTS J1647+4338, have spectra that are very typical of He CVs but no measured orbital period (Breedt et al. 2014). We include these as candidate He CVs.

sdB donor systems: this refers to mass-transferring UCBs in which the donor star is spectroscopically confirmed to be a hot subdwarf (sdB-type star). Two examples are known: ZTF J2130+4420 and ZTF J2055+4651 (Kupfer et al. 2020a,b).

BD donor systems: this refers to mass-transferring UCBs in which the donor star is a brown dwarf, that is, a dense, hydrogen-rich object with no evidence for helium enhancement relative to typical CVs, but still with P_orb < 70 min. The two known examples are SDSS J1507+5230 and WD J1540–3929.

Warm donors: two systems do not fit neatly into the above categories: ZTF J0127+5258 (P_orb = 14 min) and ATLAS J1138–5139 (P_orb = 28 min). Both systems have warm donors that are visible in the optical and have optical hydrogen lines, but cannot be spectroscopically classed as hot subdwarfs or brown dwarfs, and both have orbital periods below the range expected for He CVs. An evolutionary channel with two phases of common envelope evolution is favoured for both systems. As these systems appear somewhat similar, we put them into one class.

Unclear: six confirmed systems (and many candidates) do not have sufficient data available to classify them as one of the above subclasses. This was particularly the case where there is no measured P_orb or observed spectrum; where P_orb is highly uncertain; or for outbursting systems with 40 > P_orb < 50 min, making it difficult to distinguish between classification as an AMCVn or a He CV. These systems are not classified into any of the above subclasses in the catalogue, and are marked as Unclear in Table A.3.

4 Catalogue description

The catalogue in its present form is presented in Tables A.1–A.4. A living version of the catalogue is hosted on the website Zenodo⁹. Each iteration of the catalogue will be given a unique DOI code and all historic versions will remain accessible on Zenodo.

4.1 Description of catalogue columns

We now give an overview of the columns contained in the catalogue. For all relevant columns, the associated uncertainties are listed in a separate column with _err appended to the column name. References for specific properties are listed in columns with _ref appended, and freeform comments on specific properties are given in columns ending in _comment.

Columns Name and Other_names give various names used to refer to each object. We have attempted to favour the most commonly used name in the literature for Name. We note that the most widely used name in the literature for a given target is not always consistent with the name on the databases such as SIMBAD. Columns RA and Dec give the decimal coordinates, while Coords_hex gives the hexagesimal coordinates. Coordinates from Gaia Data Release 3 (DR3) were used where available (Section 4.2). Notes allows for free-form comments on the target that do not fit into other columns.

We include several boolean flags to help users filter their desired targets from the overall catalogue. The column Confirmed identifies targets that are confirmed to meet the critera in Section 3, as opposed to candidates. The column Has_spectrum identifies targets for which a spectrum is available, while Has_optical_hydrogen and Has_optical_donor identify systems for which the spectrum shows visible hydrogen lines or spectral lines from the donor star, respectively. The column Has_eclipses identifies targets which show partial or complete eclipses of the central white dwarf by the donor star (systems in which only the accretion disc is partially eclipsed are not included). The column Has_eROSITA identifies those for which an eROSITA crossmatch has been found. We also include boolean columns for several classes of mass-transferring UCB (see Sections 2.4 or 3.2 for further details): AM_CVn, He_CV, sdB_donor, BD_donor (brown dwarf donor), and Warm_donor.

The column Discovery_ref gives the first reference to classify the target as a UCB. The column Discovery_year gives either the year a system was first published, or (for systems with no publication) the year it was first put forward as a candidate. References to published spectra, where available, are given in Spec_ref. A cumulative list of references for each target is given in References, in addition to references for specific aspects (such as orbital period or masses) that are given in separate columns. The list of references may not be complete for some well-studied targets, but we have endeavoured to include any references which refer to specific breakthroughs for a given target.

The column Period gives the orbital period or, where that is not available, the superhump period. We note that uncertainties on the periods are not tabulated; in most cases these would be <0.01 min, and the few cases with larger uncertainties are marked in the notes and comments columns. The column Period_note gives an addendum to the period: superhump periods are marked (sh), and candidate periods or periods with large uncertainties are marked ?. The column Period_method describes how the period was measured, with options being Eclipses, Spectroscopic, or Photometric. The column Period_predicted gives periods that were predicted based on the outburst properties of the system, following the relationships of Levitan et al. (2015), only for systems with no measured periods. The column Disk_state describes the state of the accretion disc, out of High, Outburst, Low, or Direct. The last value is used in the case of systems with no accretion disc (see the discussion in Section 2.1). The column Pdot gives dP/dt for systems with such measurements, in units of ×10⁻¹² ss⁻¹.

For systems with measured mass ratios, these are given in the column Q. The column Q_method describes how the mass ratios were derived, which can be Eclipses, Spectroscopy, Superhumps, or Stage A Superhumps. Of these, the value Superhumps refers to mass ratios derived using an empirical relation that is observed between the orbital period, superhump period, and mass ratio. As discussed in Section 2.1, this relation should be used with caution, as it was derived for hydrogen-dominated systems, and is still not well tested for helium accretion discs. While several forms of the relation have been put forward, we favour the expression of McAllister et al. (2019) due to the inclusion of uncertainties on the relationship, and we recalculated all superhump-derived mass ratios using this expression to ensure homogeneity. The superhump excess (defined as ϵ = [P_sh − P_orb]/P_orb for superhump period P_sh) used is given in column Superhump_excess. In principle it is necessary to know the superhump stage (A, B, or C, referring to different parts of an outburst) during which the superhump was measured, but in most sources this is not recorded; if it is not, we assumed Stage B, which is the longest-lasting stage of the outburst. Another method in the Q_method column, Stage A superhumps, represents the method proposed by Kato & Osaki (2013) using superhumps measured during the earliest parts of an outburst; this is somewhat different from the superhump method discussed above, and so we did not recalculate these.

Several systems, mostly eclipsing systems, have direct measurements of the component stellar masses M₁ (accretor) and M₂ (donor). These are listed in the columns M1 and M2.

In several cases where multiple measurements of the same property have been published, we have selected one value as the canonical value on a case-by-case basis. We favour the purely spectroscopic masses of HM Cnc (Roelofs et al. 2010) over those of Munday et al. (2023), which were derived from the dP/dt and are dependent on a greater number of assumptions. We also favour the purely spectroscopic mass ratio of AMCVn itself (Roelofs et al. 2006b) over the superhump-derived value, but do include the component masses measured by Smak (2023) even though their derivation relies on the superhump-derived mass ratio. In each case, we have endeavoured to include all relevant references in the References column.

Fig. 6

Sky location of all known mass-transferring UCBs in equatorial coordinates. The solid black line shows the Galactic plane; there is no clear preference for UCBs to be in or out of the plane. There is a clear bias towards the northern hemisphere, where a greater number of large-sky surveys have been carried out.

4.2 Cross-match with Gaia

We performed a cross-match of all targets against Gaia DR3 (Gaia Collaboration 2022). For most targets a search radius of 5 arcsec was used. For V396Hya and GPCom, which are nearby targets with large proper motions, a wider search radius of 20 arcsec was used. From this crossmatch, we added columns containing the Gaia DR3 source IDs (Gaia_ID), G-band magnitudes (Gaia_Gmag), BP − RP colours (Gaia_BPRP), and parallaxes (Gaia_parallax). The coordinates in columns RA and Dec were updated with the Gaia coordinates for all targets with a Gaia counterpart. We also took the Gaia-based geometric distance estimates of Bailer-Jones et al. (2021), expressed as a distance median (Distance) and the 16th and 84th percentiles (Distance_16 and Distance_84). Of the objects in this catalogue, 111 out of the 123 confirmed UCBs and 19 out of the 29 candidates have Gaia crossmatch results.

5 The known sample

Here, we provide an overview and summary of the known population of mass-transferring UCBs to date. Figure 6 shows the sky position of targets in the sample. There is a clear preference for the northern hemisphere, likely because there have been more multi-object surveys in the north than in the south.

5.1 Discovery years and selection effects

Figure 1 shows the number of known mass-transferring UCBs by discovery year. Section 2.6 discusses the many large-sky surveys that have contributed to the discovery of mass-transferring UCBs. The impact of various surveys can be seen not only in the number of discoveries, but in the nature of new systems being discovered. Figure 7 shows cumulative histograms of the known mass-transferring UCBs, broken into several subclasses by accretion disc behaviour and by the presence of eclipses. The discoveries from new surveys appear around five to ten years after first light of the survey. Therefore, there is a rise in the number of low-state systems, which dominate in a magnitude-limited sample, ten years after the beginning of SDSS (e.g. Anderson et al. 2005, 2008; Rau et al. 2010), with a smaller bump several years later due to the SDSS-based follow-up survey (Roelofs et al. 2009; Carter et al. 2013a). A sharp rise in the number of outbursting systems is visible some years after the beginning of ASAS-SN and PTF (e.g. Levitan et al. 2013; Breedt et al. 2014), with a second rise shortly after the beginning of ZTF (e.g. van Roestel et al. 2021). It can also be seen that the number of eclipsing systems has increased dramatically in the years since ZTF began operating (e.g. Burdge et al. 2020; van Roestel et al. 2022). The sharp rise in low-state systems since 2020 is a combination of eclipsing systems from ZTF, X-ray discoveries from eROSITA (Rodriguez et al. 2023; Schwope et al. 2024a), and spectroscopic discoveries from SDSS-V (Inight et al. 2023a,b).

Fig. 7

Cumulative histograms of the discovery years of several different subclasses of UCB, highlighting the impact of different surveys on the make-up of the known sample. SDSS led to an increase in the discoveries of low-state systems (which should dominate in a volume-limited sample), while PTF and ASAS-SN led to rapid discoveries of outbursting systems detected photometrically. ZTF has led to a sharp rise in the number of eclipsing systems.

5.2 Distances and space density

To date, the only population-level statistic that has been measured for the mass-transferring UCB population is the space density. Several groups have estimated this for the hydrogendepleted AMCVn-type systems and found consistent results, though always with small sample sizes. Carter et al. (2013a) used a magnitude-limited sample of four AMCVn binary systems in the SDSS-III footprint to estimate the space density of AMCVn-type binaries at ρ = (5 ± 3) × 10⁻⁷ pc⁻³. Ramsay et al. (2018) used the Gaia distances of known AM CVn binary systems to estimate a lower limit, ρ > 7 × 10⁻⁸ pc⁻³. Based on the seven eclipsing AMCVn-type systems recovered from ZTF, van Roestel et al. (2022) found $ρ = 6_{- 2}^{+ 6} \times 10^{- 7} {p c}^{- 3}$ $\[\rho=6_{-2}^{+6} \times 10^{-7} \mathrm{pc}^{-3}\]$ . Rodriguez (2024) used the sample of three AMCVn-type systems recovered from the western eROSITA dataset within 150 pc to determine a space density of ρ = (5.5 ± 3.7) × 10⁻⁷ pc⁻³.

In Figure 8, we plot geometric distance estimates (Bailer-Jones et al. 2021) for nearby mass-transferring UCBs. We also plot, for comparison, a predicted distribution based on the space density estimate of van Roestel et al. (2022). The distribution was calculated using the method of Pretorius et al. (2007), assuming a Galactic disc with a scale height 300 pc, which is consistent with the best fit for CVs (Pala et al. 2020).

It appears that our distance distribution is consistent with the van Roestel et al. (2022) density estimate and follows the expected Galactic distribution out to ≈300 pc. This suggests that our completeness is relatively high within 300 pc and falls significantly beyond that limit. We note, however, that systems within 300 pc have continued to be discovered recently (van Roestel et al. 2022; Inight et al. 2023b; Rodriguez et al. 2023). When we consider also that the southern hemisphere remains under-explored relative to the northern hemisphere (Figure 6), it seems likely that more systems are waiting to be discovered within 300 pc.

Fig. 8

Cumulative histogram of the distances to known mass-transferring UCBs. Also plotted is the expected distribution for the local space density measured by van Roestel et al. (2022), extrapolated using a Galactic model with a scale height of 300 pc. The true systems approximately follow the prediction of van Roestel et al. (2022) in the range 150–300 pc, after which incompleteness becomes much more severe. There appears to be an excess at short distances when compared to the prediction (two systems within 100 pc where only one would be predicted), but given the small numbers this is not statistically significant.

5.3 Orbital periods

Orbital period is the most widely measured physical property of the UCBs in this catalogue: 101 out of the 123 confirmed UCBs have a measured orbital or superhump period. The most common method by which orbital periods are determined is photometric measurement of a superhump or an orbital signature that is not related to eclipses (42 systems), followed by spectroscopic measurement (33 systems) and timing of eclipses (25 systems; this includes systems with partial disc eclipses, even when the central white dwarf is not eclipsed).

In Figure 2, we show the period distribution of the known systems. When considering all known systems, the period distribution is strongly affected by the selection biases discussed in Section 5.1; most notably, the prominent peak of outbursting systems, which are preferentially found through photometric variability surveys. In principle it should be possible to study the underlying period distribution of the population while modelling the selection effects of defined subsets of the sample, such as volume-limited or eclipsing subsamples.

Fig. 9

Absolute magnitudes of targets in this catalogue as a function of orbital period. We have only included targets for which the Gaia parallax is detected with 5σ confidence. The grey solid lines show the white dwarf magnitudes predicted by Bildsten et al. (2006) for white dwarf donors of two different levels of initial entropy, and the grey dotted line shows the accretion disc magnitude predicted by Nelemans et al. (2004). AMCVn-type systems in the low state follow an approximately linear relationship with orbital period (fitted by the blue dashed line). Other types of systems are generally brighter, due to extra light from the donor star and/or the accretion disc. As was previously discussed by Ramsay et al. (2018), even low-state systems at long orbital periods are systematically brighter than the model predictions.

5.4 Magnitudes and colours

Ramsay et al. (2018) noted that the absolute magnitude of AMCVn-type systems depends strongly on their orbital period, reflecting the correlation between mass transfer rate and orbital period and the resulting changes in accretion behaviour described in Section 2.1. In Figure 9, we show the absolute G-band magnitudes of our sample as a function of orbital period, using distances from Bailer-Jones et al. (2021). We have removed any targets for which the Gaia parallax is detected with less than 5σ significance. We also plot the expected magnitude of the accretion disc (Nelemans et al. 2004) and the accreting white dwarf with an assumed white dwarf donor and two possible mass transfer rates (Bildsten et al. 2006). Both of these theoretical predictions have been converted from the V to G pass-bands¹⁰ using the median BP − RP colour of AMCVn-type binaries, although the correction was only 0.02 magnitudes.

Several patterns can be observed among the classical AMCVn-type systems. Firstly, the brightest systems are high-state and direct-impact systems (yellow and brown stars) with P_orb ≲ 20 min, where the emission is dominated by accretion luminosity. At P_orb ≳ 25 min the accretor-dominated AMCVn-type systems (blue circles) are close to the Bildsten et al. (2006) prediction for the accretor luminosity, but are slightly elevated. This is consistent with the finding that estimates of accretor temperature are elevated above model-derived predictions (Macrie et al. 2024). This elevation may be the result of their non-DB atmospheres or of the low donor masses, and hence low donor entropy and low mass-transfer rates, assumed by Bildsten et al. (2006). Recently it has also been suggested that additional orbital angular momentum loss, such as magnetic braking, may speed up the period evolution of AMCVn-type binaries (Belloni et al. 2024; Maccarone et al. 2024), which may then affect the relation between orbital period and accretor luminosity. In the 20–25 min period range, frequent outbursts may bias the measured magnitudes upwards, resulting in the apparent transition region between the low-state and high-state systems. Ramsay et al. (2018) gave a detailed discussion of individual AMCVn-type systems on this plot, including the notable outliers ASASSN-14ei (P_orb = 42 min) and SDSS J0804+1616 (44.5 min).

Other types of mass-transferring UCBs do not follow these same patterns. The two sdB donor systems (Kupfer et al. 2020a,b) are dominated by their donor luminosity and so are substantially brighter. The He CVs (purple diamonds) are generally brighter than classical AMCVns at the same orbital period, due to their higher mass transfer rates and (in some cases) the contribution of light from their donor stars. Donor contribution is also significant in the magnitude of ATLAS J1138–5139 8 (a warm-donor binary with P_orb = 28 min). SDSS J1507+5230 (a brown dwarf donor, P_orb = 67 min) is also brighter than an AMCVn-type binary at that orbital period, but has no measurable contribution in the optical from its donor star (Littlefair et al. 2007), and so its brightness is likely driven by a higher mass transfer rate.

For future analyses, it may be useful to parametrise the dependence of absolute magnitude on orbital period. For low-state or outbursting classical AMCVn-type systems (blue circles), the dependence is well described (for orbital periods of 25–70 min) by

$M_{G} (P_{orb}) = 7.9 (2) + 0.076 (4) (\frac{P_{orb}}{m i n}) m a g .$ $\[M_G\left(P_{\text {orb }}\right)=7.9(2)+0.076(4)\left(\frac{P_{\text {orb }}}{\mathrm{min}}\right) \mathrm{mag}.\]$ (1)

The numbers in parentheses are uncertainties on the last digit of the corresponding value. To arrive at this fit we performed a sigma-clipping routine on the low-state AMCVn-type systems, iteratively removing the most significant outlier and repeating the fit until all remaining points were within 3σ of the best-fit line. This best-fit line is plotted as a blue dashed line in Figure 9.

In Figure 10, we plot the Gaia colour-magnitude diagram of the confirmed AMCVn-type (completely hydrogen depleted) binary systems. A Z-shaped dependence of colour and magnitude on orbital period was noted by van Roestel et al. (2022). Disc-dominated systems (P_orb ≲ 20 min) are generally bluest at the shortest periods, and become redder towards P_orb ≈ 20 min. Then, as they transition to become accretor-dominated, they become bluer again, until they merge with the white dwarf sequence at P_orb ≈ 30–40 min. Towards P_orb ≳ 40 min the systems cool and redden in parallel with white dwarf cooling tracks.

Roelofs et al. (2009) and Carter et al. (2013a) used colour selection to identify candidate UCBs for spectroscopic followup. Ten of the targets in this catalogue (nine AMCVn-type systems and one He CV) were found by that survey, making it a significant contributor to the known sample. The colour cuts were defined by Roelofs et al. (2009) to include all AMCVn-type binaries known at that time, and the selection was assumed to be complete in terms of colour by Carter et al. (2013a) for the space density analysis discussed in 5.2. The discovery of several AMCVn-type systems outside the original selection box led Carter et al. (2014a) to propose an expanded set of selection criteria, but follow-up spectroscopy was not obtained using the new criteria.

In Figure 11, we plot the SDSS u′g′r′ colours for all targets in this catalogue within a limiting magnitude of g′ < 20.5, using the cross-match performed by Gentile Fusillo et al. (2021). We also plot the selection boxes of Roelofs et al. (2009) and Carter et al. (2014a). Of the known AMCVn-type systems, 29 have SDSS u′g′r′ magnitudes (not including a further two which are outside the limits of this plot, presumably due to unreliable colour measurements or contamination from nearby stars). 25 of those are selected by the updated criteria of Carter et al. (2014a), while only 17 are selected by the original cuts of Roelofs et al. (2009) that were implemented in the follow-up survey. While not the intended target of that survey, several He CVs are selected by the updated cuts while none are selected by the original cuts. We therefore emphasise that future colour-based searches for mass-transferring UCBs should use the Carter et al. (2014a) criteria or similar, rather than the more conservative Roelofs et al. (2009) criteria.

Fig. 10

Colour–magnitude diagram of confirmed AMCVn-type binaries in this catalogue. Points are coloured according to their orbital periods. Also plotted are volume-limited samples of main sequence stars and white dwarfs, drawn randomly from Gaia DR3. The AM CVntype systems follow a Z-shaped pattern pointed out by van Roestel et al. (2022): short period systems are elevated above the white dwarf sequence because they are dominated by the accretion disc, while longer-period systems follow a white dwarf cooling sequence.

5.5 Masses and radii

A minority of mass-transferring UCBs have measured stellar masses and radii, primarily measured by eclipse light curve fitting. In Figure 12, we plot the measured masses of accretors across all systems in this catalogue. All of these accretor masses are measured from eclipses, except for the spectroscopically derived mass of AMCVn itself (Smak 2023).

The majority of accretor masses are consistent with the typical 0.83 ± 0.17 M_⊙ mass range observed across accreting white dwarfs in CVs (Pala et al. 2020). As discussed in Section 2.3, ATLAS J1138–5139 has an unusually high-mass accretor, and is a promising candidate for a Type Ia supernova progenitor. The two systems with sdB donor stars, ZTF J2130+4420 and ZTF J2055+4651, are more consistent with the typical ≈0.6 M_⊙ masses seen in non-accreting white dwarfs (e.g. O’Brien et al. 2024). These systems are thought to be in an early stage of mass transfer (Bauer & Kupfer 2021) and have likely not yet accreted a significant helium envelope. ZTF J1813+4251, a He CV which is thought to be passing through a transitional stage between the longer-period CVs and the AMCVn-type binaries (Burdge et al. 2022), also has a low-mass accretor more typical of a non-accreting white dwarf.

In Figure 13, we plot all of the UCBs that have measured donor masses. In the left column we plot measured units (P_orb and M₂), while in the right column we plot M₂ and R₂, following the plots of Solheim (2010), Green et al. (2018a,b, 2020), and van Roestel et al. (2022). There is a well-established relationship between the orbital period and mean density of a Roche-lobe filling donor star (Faulkner et al. 1972). As the period is tightly constrained for all systems on this plot, the uncertainties on M₂ and R₂ are strongly correlated, and we plot the error bars diagonally along lines of constant period to show this.

There are many more systems with measurements of mass ratio (q = M₂/M₁) than with direct measurements of M₂. Where only q is measured, we have adopted M₁ = 0.83 ± 0.17 (Pala et al. 2020), and estimated M₂ in this way. As discussed in previous sections, in the case of systems for which q was derived from Stage B or C superhumps, we have re-derived q using the relations of McAllister et al. (2019) to ensure consistency.

We also plot evolutionary tracks from several different sources. In the upper panels, we plot white dwarf and helium star donor models. For the white dwarf donor channel, we use the models of Wong & Bildsten (2021). The vertical distribution of tracks corresponds to white dwarf donors with varying levels of thermal inflation of the donor at the onset of mass transfer. Binaries with wider separations at the end of the common envelope phase have longer inspiral times preceding the onset of mass transfer and hence more time for the donor to cool, resulting in more compact donor stars. Because of the uncertainties outlined in that paper about the subsequent cooling rate of the donor star during mass transfer, we plot two sets of tracks: one set with nominal donor cooling rates, and a further set of tracks with equivalent initial conditions but for which donor cooling was not allowed. The latter case causes donors to remain at higher levels of inflation as they evolve towards longer orbital periods, which helps to explain the high-mass and large-radius donors observed at longer periods.

We also plot the helium star donor track modelled by Wong & Bildsten (2021). Post-bounce, helium star donors move approximately parallel to white dwarf donors in mass-radius space, with larger radii for a given mass. To demonstrate the pre-bounce evolution of the helium star channel and the connection to the two sdB-donor systems, we plot two evolutionary tracks for sdB-donor binary systems from Rajamuthukumar et al. (2024). The models of Rajamuthukumar et al. (2024) were terminated at the first helium nova eruption, due to uncertainties around the modelling of nova.

In the lower panels, we plot models of the evolved CV formation scenario, using models with enhanced magnetic braking by Belloni & Schreiber (2023) and Sarkar et al. (2023a). Although the adopted prescriptions for magnetic braking are different between the two works, the distributions of the tracks are qualitatively similar. Earlier evolved CV models without enhanced magnetic braking (e.g. Goliasch & Nelson 2015) were generally limited to P_orb ≳ 40 min and more massive donors.

The AMCVn-type systems (systems with no detected hydrogen; the blue circles in Figure 13) form a sequence in donor mass-period or mass-radius space. Under the white dwarf or helium star donor channels, this is an evolutionary sequence, running parallel with the evolutionary tracks of Wong & Bildsten (2021) from short to long orbital periods. In evolved CV models with enhanced magnetic braking (Belloni et al. 2024; Sarkar et al. 2023a), the AMCVn sequence is again an evolutionary sequence which systems follow after going through a period bounce, with more inflated systems generally having undergone the period bounce at longer orbital periods.

He CVs (the magenta points in Figure 13) do not follow the same sequence as classical AMCVn binaries, but span a range of donor masses with periods always in the range of 50–70 min. At P_orb ≳ 50 min and M₂ ≲ 0.05M_⊙, the AMCVn and He CV regions of parameter space overlap. Under the evolved CV scenario, some or all of these He CVs can be understood as progenitors of the hydrogen-depleted AMCVn-type systems.

Each set of model tracks is consistent with at least some observed systems, and most observed systems can be explained by multiple evolutionary scenarios. Tracks from the white dwarf or helium star donor scenarios can generally explain most of the classic AMCVn-type systems as well as the sdB-donor binaries and other binaries with warm donors at short orbital periods (e.g. ZTF J0127+5258 and ATLAS J1138–5139), but still struggle to explain some of the more inflated donors, especially at longer periods, and cannot explain most He CVs. Evolutionary tracks from the evolved CV scenario can again explain most of the AMCVn-type systems, as well as most He CVs, but struggle to explain some high-mass donors at short periods. It seems likely that there are two or three evolutionary scenarios at play in the formation of the AMCVn-type binaries.

In most cases it is not possible to determine for any given system which formation channel it must have descended from, as the measurable properties that can arise from different channels have a significant degree of overlap. Instead, the most promising avenue to test the proposed formation channels is on a population level, comparing observed population statistics such as space density, or the distribution of orbital periods and masses, against those predicted by different channels.

Fig. 11

Colour-colour plot of targets in this sample with SDSS u′g′r′ colours and g′ < 20.5. A further four points (two AMCVn-type and two He CVs) are not shown because they would be outside the panel. The grey two-dimensional histogram shows the distribution of a sample of white dwarfs (Gentile Fusillo et al. 2021). The black dotted line shows the original colour–colour selection box of Roelofs et al. (2009), and the black dashed line shows the updated selection criteria of Carter et al. (2014a). A number of mass-transferring UCBs are missed by the older selection criteria but selected by the newer criteria.

Fig. 12

Measured masses of the accreting components of the binaries in this catalogue. The marker shapes and colours match those in Figure 9. All masses plotted here were measured by eclipses, except AM CVn itself which is measured spectroscopically (Smak 2023). Also labelled are the typical masses for white dwarfs in isolation and in mass-transferring binaries, and the approximate mass range for which typical Type Ia supernovae are expected under the double-detonation scenario. We note that ATLAS J1138–5139 is the only binary with sufficient mass for a typical Type Ia supernova. The two accretors with sdB-type donors (green carets) and the pre-bounce He CV (magenta diamond) are lower in mass than the rest of the distribution, presumably because their accumulated mass from accretion is still low; their masses are more typical of isolated white dwarfs, while the majority of systems have masses more typical of accreting white dwarfs.

Fig. 13

Measured orbital periods and donor star masses and radii for mass-transferring UCBs in the catalogue. In the left panels, we plot the observed properties: orbital period and donor mass. In the right panels, we plot donor mass and radius. Uncertainties in the right panels are diagonal because of the strong constraint on donor density that arises from the orbital period. The two systems plotted with red squares are the brown dwarf donor SDSS J1507+5230 and the 30 min supernova progenitor candidate ATLAS J1138-5139. We also plot several evolutionary tracks from different scenarios, as discussed in the text: the upper panels show evolutionary models of the white dwarf and helium star donor tracks, while the lower panels show evolutionary models from the evolved CV scenario. The systems without hydrogen (AMCVn-type binaries) follow a sequence from short to long orbital periods, while He CVs with hydrogen follow a different sequence from low to high masses. In general, any formation scenario can explain at least some fraction of the known systems, and no single formation scenario can explain every system.

6 Summary

We have presented a catalogue of mass-transferring UCBs, including all known binary systems with orbital periods <70 min containing accreting white dwarfs. The catalogue is presented in the tables in Appendix A. A living version of the catalogue will be hosted on the Zenodo platform and updated regularly.

Given their compact orbits, these binaries necessarily consist of two evolved and dense objects. In almost all cases the accreted material is partially or completely hydrogen-depleted. The majority of systems in the catalogue belong to the class called AMCVn-type binaries, which we define as having no detectable hydrogen, while a minority belong to the partially hydrogen-depleted class called He CVs. The two classes are thought to be evolutionarily related under some formation scenarios, and occupy similar but distinct regions of parameter space in terms of orbital periods, outburst behaviours, masses and radii. Several other unusual binary systems also meet the criteria for inclusion in the catalogue. The literature overview in Section 2 may be a useful resource for those who are new to the field.

We have given an overview of a number of observable properties of the known sample of mass-transferring UCBs. The sample shows severe incompleteness effects at distances ≳300 pc, but within 300 pc agrees well with previous observational space density estimates. The distribution of orbital periods shows strong biases, particularly towards period ranges that exhibit dwarf nova outbursts.

We have presented an overview of the stellar masses and radii measured to date. In particular, the donor masses and radii can give insight into the formation history of a binary system. Of the three proposed formation channels, evolutionary tracks for any channel could explain most of the population, and no channel can explain every mass-transferring UCB. In order to better understand the formation history of these systems, and the relative importance of the proposed formation channels, a population-level comparison between models and observed systems may be the next step for the field. This will require effort on both the observational front (continuing to characterise the growing sample of known systems) and the theoretical (predicting population-level distributions of parameters that can be compared to the systems in this catalogue).

Data availability

Full Tables A.1–A.4 are available at the CDS via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/700/A107

Acknowledgements

This paper is dedicated to the memory of Tom Marsh, a dearly missed mentor, colleague and friend. The authors are grateful to the organisers of the 2022 Kavli Institute of Theoretical Physics workshop on white dwarfs, where this paper was first planned, and to the organisers of the 2023 AMCVn 5 workshop at Armagh Observatory, where many useful discussions took place. The authors would like to thank the anonymous referee for their helpful comments and feedback. We are grateful to Evan Bauer, Diogo Belloni, Anton Biryukov, Boris Gänsicke, Stuart Littlefair, Gavin Ramsay, Liliana Rivera-Sandoval, Matthias Schreiber, Tony Rodriguez, and Arnab Sarkar for helpful comments and discussions. We are also grateful to James Munday and Anwesha Sahu for providing feedback on early versions of the catalogue. MJG thanks Mitch Begelman and the JILA department at the University of Colorado, Boulder, for generously providing office space at which much of this paper was written. MJG acknowledges support from the European Research Council through ERC Advanced Grant No. 101054731, from the National Aeronautics and Space Administration under grants 80NSSC24K0436, 80NSSC22K0479, and 80NSSC24K0380, and from the National Science Foundation under grant AST-2205736. This publication is part of the project “The life and death of white dwarf binary stars” (VI.Veni.212.201) of the research programme NWO Talent Programme Veni Science domain 2021 which is financed by the Dutch Research Council (NWO). TLSW acknowledges and thanks support by the National Science Foundation through grant PHY-2309135, and by the Gordon and Betty Moore Foundation through grant GBMF5076. This work made use of the software packages TOPCAT, PYTHON, NUMPY, MATPLOTLIB, SCIPY, ASTROPY, and TRM.

Appendix A Catalogue

Tables A.1–A.4 contain a simplified overview of the catalogue of all known mass-transferring UCBs. The full catalogue, including many columns not shown here, is available in its current form at the Centre de Données astronomiques de Strasbourg (CDS). It is also available on Zenodo, in a version that will be updated regularly, at https://doi.org/10.5281/zenodo.8276712.

Table A.1

List of confirmed AMCVn-type systems (without detected hydrogen) in the catalogue.

Table A.2

cont.—List of confirmed AMCVn-type systems (without detected hydrogen) in the catalogue.

Table A.3

List of non-AMCVn confirmed UCBs in the catalogue.

Table A.4

List of candidate UCBs in the catalogue.

Appendix B References

Table B.1 contains the reference keys used in the catalogue and links them to the full reference listed in the reference section.

Table B.1

References used in the catalogue.

Table B.2

References used in the catalogue, cont.

Table B.3

References used in the catalogue, cont.

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¹

Having been identified as a ‘decidedly blue’ star by Humason & Zwicky (1947), leading to the star’s earlier name of HZ 29.

²

Over five decades later, the same Smak (2023) combined his original 1962 data with more recent observations to measure the period derivative of the binary system.

³

More properly, most mass-transferring UCBs experience superoutbursts more often than regular outbursts, though both of these phenomena as well as a variety of other behaviours are observed (e.g. Duffy et al. 2021; Pichardo Marcano et al. 2021). Superoutbursts are somewhat brighter and longer in duration than regular outbursts. For the purposes of this work we do not emphasise the distinction.

⁴

See also Deshmukh et al. (2024) for a discussion of a comparable WD+sdB binary that is not currently mass-transferring, and Rajamuthukumar et al. (2024) for a detailed discussion of novae and supernovae in WD+sdB binaries.

⁵

https://research.iac.es/proyecto/compactos/UltraCompCAT/

⁶

A black hole may also be possible, but none has been found so far.

⁷

http://www.kusastro.kyoto-u.ac.jp/vsnet/

⁸

https://astronomerstelegram.org/

⁹

Current version: https://zenodo.org/records/15116793 The following link will always resolve to the latest version: https://doi.org/10.5281/zenodo.8276712

¹⁰

https://gea.esac.esa.int/archive/documentation/GDR2/Data_processing/chap_cu5pho/sec_cu5pho_calibr/ssec_cu5pho_PhotTransf.html

All Tables

Table A.1

List of confirmed AMCVn-type systems (without detected hydrogen) in the catalogue.

In the text

Table A.2

cont.—List of confirmed AMCVn-type systems (without detected hydrogen) in the catalogue.

In the text

Table A.3

List of non-AMCVn confirmed UCBs in the catalogue.

In the text

Table A.4

List of candidate UCBs in the catalogue.

In the text

Table B.1

References used in the catalogue.

In the text

Table B.2

References used in the catalogue, cont.

In the text

Table B.3

References used in the catalogue, cont.

In the text

All Figures

	Fig. 1 Rise in the number of known mass-transferring UCBs since Smak (1967) first suggested that the variable star AMCVn may be a 17 min mass-transferring binary system. Also plotted are the times at which several large sky surveys went live, contributing to the rapid growth in the known sample since the turn of the century.
In the text

Fig. 2

Period distribution of all known mass-transferring UCBs, colour-coded according to their accretion disc behaviour in the case of AMCVn-type systems (those without detected hydrogen). The He CVs (systems with detected hydrogen) and other classes are coloured separately. The accretion disc behaviour clusters by orbital period, but with some regions of overlap between the different subclasses. He CVs are typically found at periods ≳50 min. We note that most He CVs do show outbursts, while systems in the same period range without hydrogen typically do not. The orbital period distribution is strongly influenced by selection effects, most notably the fact that it is easier to find outbursting systems.

In the text

	Fig. 3 Schematic diagram of the proposed evolutionary paths by which mass-transferring UCBs can form, beginning with a solar-type binary system and passing through three proposed formation channels. The labels highlight the white dwarf donor channel (A), helium star donor channel (B), and evolved cataclysmic variable channel (C). See text for details of the channels.
In the text

Fig. 4

Period distributions of the overall population of CVs (Ritter & Kolb 2003, seventh edition, updated in 2014), CVs with evolved donors (provided by B. Gänsicke, priv. commun. 2025), and AMCVn-type binaries (systems without detectable hydrogen, this work). Magnetic CVs and CVs with orbital periods below the non-degenerate period minimum were excluded from the CV period distribution. The orbital periods of evolved CVs cover a broad range; the shortest are ≲70 min, and therefore fall into the period range of this catalogue.

In the text

	Fig. 5 Flow chart demonstrating the process by which targets are included in or excluded from the catalogue.
In the text

	Fig. 6 Sky location of all known mass-transferring UCBs in equatorial coordinates. The solid black line shows the Galactic plane; there is no clear preference for UCBs to be in or out of the plane. There is a clear bias towards the northern hemisphere, where a greater number of large-sky surveys have been carried out.
In the text

Fig. 7

Cumulative histograms of the discovery years of several different subclasses of UCB, highlighting the impact of different surveys on the make-up of the known sample. SDSS led to an increase in the discoveries of low-state systems (which should dominate in a volume-limited sample), while PTF and ASAS-SN led to rapid discoveries of outbursting systems detected photometrically. ZTF has led to a sharp rise in the number of eclipsing systems.

In the text

Fig. 8

Cumulative histogram of the distances to known mass-transferring UCBs. Also plotted is the expected distribution for the local space density measured by van Roestel et al. (2022), extrapolated using a Galactic model with a scale height of 300 pc. The true systems approximately follow the prediction of van Roestel et al. (2022) in the range 150–300 pc, after which incompleteness becomes much more severe. There appears to be an excess at short distances when compared to the prediction (two systems within 100 pc where only one would be predicted), but given the small numbers this is not statistically significant.

In the text

Fig. 9

Absolute magnitudes of targets in this catalogue as a function of orbital period. We have only included targets for which the Gaia parallax is detected with 5σ confidence. The grey solid lines show the white dwarf magnitudes predicted by Bildsten et al. (2006) for white dwarf donors of two different levels of initial entropy, and the grey dotted line shows the accretion disc magnitude predicted by Nelemans et al. (2004). AMCVn-type systems in the low state follow an approximately linear relationship with orbital period (fitted by the blue dashed line). Other types of systems are generally brighter, due to extra light from the donor star and/or the accretion disc. As was previously discussed by Ramsay et al. (2018), even low-state systems at long orbital periods are systematically brighter than the model predictions.

In the text

Fig. 10

Colour–magnitude diagram of confirmed AMCVn-type binaries in this catalogue. Points are coloured according to their orbital periods. Also plotted are volume-limited samples of main sequence stars and white dwarfs, drawn randomly from Gaia DR3. The AM CVntype systems follow a Z-shaped pattern pointed out by van Roestel et al. (2022): short period systems are elevated above the white dwarf sequence because they are dominated by the accretion disc, while longer-period systems follow a white dwarf cooling sequence.

In the text

Fig. 11

Colour-colour plot of targets in this sample with SDSS u′g′r′ colours and g′ < 20.5. A further four points (two AMCVn-type and two He CVs) are not shown because they would be outside the panel. The grey two-dimensional histogram shows the distribution of a sample of white dwarfs (Gentile Fusillo et al. 2021). The black dotted line shows the original colour–colour selection box of Roelofs et al. (2009), and the black dashed line shows the updated selection criteria of Carter et al. (2014a). A number of mass-transferring UCBs are missed by the older selection criteria but selected by the newer criteria.

In the text

Fig. 12

Measured masses of the accreting components of the binaries in this catalogue. The marker shapes and colours match those in Figure 9. All masses plotted here were measured by eclipses, except AM CVn itself which is measured spectroscopically (Smak 2023). Also labelled are the typical masses for white dwarfs in isolation and in mass-transferring binaries, and the approximate mass range for which typical Type Ia supernovae are expected under the double-detonation scenario. We note that ATLAS J1138–5139 is the only binary with sufficient mass for a typical Type Ia supernova. The two accretors with sdB-type donors (green carets) and the pre-bounce He CV (magenta diamond) are lower in mass than the rest of the distribution, presumably because their accumulated mass from accretion is still low; their masses are more typical of isolated white dwarfs, while the majority of systems have masses more typical of accreting white dwarfs.

In the text

Fig. 13

Measured orbital periods and donor star masses and radii for mass-transferring UCBs in the catalogue. In the left panels, we plot the observed properties: orbital period and donor mass. In the right panels, we plot donor mass and radius. Uncertainties in the right panels are diagonal because of the strong constraint on donor density that arises from the orbital period. The two systems plotted with red squares are the brown dwarf donor SDSS J1507+5230 and the 30 min supernova progenitor candidate ATLAS J1138-5139. We also plot several evolutionary tracks from different scenarios, as discussed in the text: the upper panels show evolutionary models of the white dwarf and helium star donor tracks, while the lower panels show evolutionary models from the evolved CV scenario. The systems without hydrogen (AMCVn-type binaries) follow a sequence from short to long orbital periods, while He CVs with hydrogen follow a different sequence from low to high masses. In general, any formation scenario can explain at least some fraction of the known systems, and no single formation scenario can explain every system.

In the text

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