eROSITA selection of new period-bounce cataclysmic variables

First follow-up confirmation using TESS and SDSS

¹ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen Sand 1 72076 Tübingen, Germany
² Leibniz-Institut für Astrophysik Potsdam (AIP) An der Sternwarte 16 14482 Potsdam, Germany
³ Department of Astronomy, University of Washington Box 351580 Seattle WA 98195, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 September 2025
Accepted: 21 January 2026

Abstract

Context. Between 40% and 80% of cataclysmic variables (CVs) are expected to have evolved past the period-minimum and contain a degenerate donor. However, observational surveys for CVs have only been able to detect a few of these highly evolved “period-bouncers”, most likely due to the intrinsic faintness associated with their predicted low mass accretion rates.

Aims. We searched for new period-bouncers in optical white dwarf (WD) catalogs. Our findings will establish if the missing population of this underrepresented type of CVs can be identified through these existing catalogs.

Methods. We have produced an initial selection of high-likelihood period-bounce candidates based on our multiwavelength period-bouncer scorecard and selection cuts including X-ray data from the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spektrum-Roentgen-Gamma spacecraft (SRG). We have laid out a clear path based on three main requirements (classification as a CV, determination of an orbital period, and detection of a very late-type donor) that should result in the confirmation of several of these candidates, and that will serve as a reference for future searches of period-bouncers.

Results. Our selection method of identifying new period-bouncers in WD catalogs successfully retrieves already known period-bouncers found in the Gaia DR3 WD catalog, and also identifies five new period-bouncers classified in the literature only as CVs, and 137 high-likelihood period-bounce candidates that were classified in the literature only as single WDs. Our path for confirming these candidates has already produced its first successful result with the confirmation of GALEX J125751.4-283015 as a new period-bouncer. Several other candidates have already fulfilled at least one of our three requirements, making their future confirmation likely.

Conclusions. Our search for period-bouncers using the X-ray eROSITA emission of objects in optical WD catalogs has led to the confirmation of six new period-bouncers identified from the Gaia DR3 WD catalog (five previously known CVs and one WD candidate), an 18% increase that brings the present population to 39 systems. Both the selection method for period-bounce candidates and the confirmation path that we outlined will aid in future searches for new period-bounce candidates, contributing to the goal of resolving the discrepancy between the high predicted number of period-bouncers and the low number of these systems successfully observed to date.