Revisiting the extremely long-period cataclysmic variables V479 Andromedae and V1082 Sagittarii

¹ Universidad Nacional Autónoma de México, Instituto de Astronomía, Aptdo Postal 6106, Ensenada, 22860 Baja California, Mexico
² São Paulo State University (UNESP), School of Engineering and Sciences, Guaratinguetá, Brazil
³ European Southern Observatory, Karl Schwarzschild Straße 2, D-85748 Garching, Germany
⁴ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Department of Physics and Astronomy, Texas Tech University, 2500 Broadway, Lubbock, TX 79409, USA
⁶ Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
⁷ American Association of Variable Star Observers (AAVSO), 185 Alewife Brook Pkwy, Suite 410, Cambridge, MA 02138, USA
⁸ Observadores de Supernovas (ObSN), Observatorio Cerro del Viento, MPC I84, Pl. Fernández Pirfano 3-5A, Badajoz 06010, Spain
⁹ Department of Astronomy, University of Washington, Seattle, WA 98195, USA
¹⁰ INAF - Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
¹¹ Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Valparaíso, Chile
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Eureka Scientific, Inc., 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
¹⁴ Silesian University of Technology, Akademicka 16, Gliwice, Poland
¹⁵ Observadores de Supernovas (ObSN), Observatorio Major de Dalt, C/ Major 54-2, Sant Celoni, Barcelona 08470, Spain
¹⁶ Abbey Ridge Observatory, 45 Abbey Rd, Stillwater Lake, NS B3Z1R1, Canada
¹⁷ Observadores de Supernovas (ObSN), Observatorio Magalofes, MPC Y85, C/ Feal 20, Fene, A Coruña 15509, Spain
¹⁸ Madrona Peak Observatory, 4635 Shadow Grass Dr, Katy, TX 77493, USA
¹⁹ Supra Solem Observing, SkiesAway Remote Observatory, Bradley, CA, USA
²⁰ Observadores de Supernovas (ObSN), Observatorio El Llagarń, C/ Rio Dobra 18, Oviedo 33010, Spain
²¹ Vereniging voor Sterrenkunde, Zeeweg 96, 8200 Brugge, Belgium
²² Observadores de Supernovas (ObSN), Observatorio Estelia, MPC Y90, C/ Ladines 12, Ladines, Asturias 33993, Spain
²³ Observadores de Supernovas (ObSN), Observatorio LaVara, MPC J38, Barrio La Vara s/n, Valdés, Asturias 33784, Spain
²⁴ Observadores de Supernovas (ObSN), Observatorio Uraniborg C/ Antequera, 8, 41400 Écija, Sevilla, Spain
²⁵ Mittelman ATMoB Observatory/Amateur Telescope Makers of Boston, Inc. 99 College Ave. Arlington, Massachusetts 02474, USA
²⁶ Dept. Chemistry and Physics, College of Arts and Sciences, Western Carolina University, Apodaca 116B, 1 University Way, Cullowhee, NC 28723, USA
²⁷ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK 73019, USA
²⁸ Vereniging Voor Sterrenkunde (VVS), Zeeweg 96, 8200 Brugge, Belgium
²⁹ Groupe Européen d’Observations Stellaires (GEOS), 23 Parc de Levesville, 28300 Bailleau l’Evêque, France
³⁰ Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV), Munsterdamm 90, 12169 Berlin, Germany
³¹ American Association of Variable Star Observers (AAVSO), bul. Rakovski 39-V-14, 6400 Dimitrovgrad, Bulgaria
³² Pelagia-Eleni observatory, Glyfada, Athens, Greece
³³ CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Av. Inte. Güiraldes 2620, C1428ZAA Buenos Aires, Argentina
³⁴ Universidade Estadual Paulista “Júlio de Mesquita Filho”, UNESP, Campus of Guaratinguetá, Av. Dr. Ariberto Pereira da Cunha, 3334 – Pedregulho, Guaratinguetá SP 12516-410, Brazil
³⁵ Observadores de Supernovas (ObSN), Observatorio Mazariegos, MPC Z50, Mazariegos, Palencia 34170, Spain
³⁶ Observadores de Supernovas (ObSN), Maritime Alps Observatory, MPC K32, Via Mellana 26, Cuneo 12100, Italy
³⁷ Observadores de Supernovas (ObSN), Observatorio Carpe Noctem, MPC I72, Paseo de la maliciosa 11, Collado Mediano, Madrid 28027, Spain
³⁸ Sección de Estrellas Variables – Centro de Observadores del Espacio – Liga Iberoamericana de Astronomía. (S.E.V.-C.O.D.E./L.I.A.D.A.), Av. Almirante Guillermo Brown Nro. 4998, Costanera Oeste, 3000 Santa Fe, Argentina
³⁹ MCD Observatory, 23 Langlois G0K1H0, Canada
⁴⁰ Fundación Astronomía Sigma Octante, Pasaje Man Cesped 592, 0301 Cochbamba, Bolivia
⁴¹ Observadores de Supernovas (ObSN), Observatorio de Sencelles, MPC K14, Camí de Sonfred 1, Sencelles, Islas Baleares 07140, Spain
⁴² American Association of Variable Star Observers (AAVSO), 5 Inverness Way, Hillsborough, CA 94010, USA
⁴³ Observadores de Supernovas (ObSN), Observatorio Montcabrer, MPC 213, C/ Jaume Balmes 24, Cabrils, Barcelona 08348, Spain
⁴⁴ American Association of Variable Star Observers (AAVSO), Via Gioacchino Rossini 2b, 95030 Pedara, Italy
⁴⁵ Nedlands Observatory, 35 Viewway, Nedlands, WA 6009, Australia
⁴⁶ Toadhall Observatory, 2095, Bakery Hill, Victoria 3354, Australia
⁴⁷ Instituto Nacional de Pesquisas Espaciais (INPE/MCTI), Av. dos Astronautas, 1758, São José dos Campos, SP, Brazil
⁴⁸ Instituto de Astrofísica de Canarias, C/O Vía Láctea s/n, 38200 La Laguna, Spain
⁴⁹ Observadores de Supernovas (ObSN), Observatorio de Masquefa, MPC 232, Av. Can Marcet 41, Masquefa, Barcelona 08783, Spain
⁵⁰ Observadores de Supernovas (ObSN), Cal Maciarol mòdul 8 Observatory, MPC A02, Masia Cal Maciarol, Camí de lÓbservatori s/n, Ager, Lleida 25691, Spain
⁵¹ British Astronomical Association Variable Star Section, P.O. Box 702 Tonbridge TN9 9TX, UK
⁵² Dark Sky New Mexico, 30 Washburn Rd., Animas, NM 88020, USA
⁵³ Lake County Astronomical Association, 28478 W. Brandenburg Road, Ingleside, IL 60041, USA
⁵⁴ Southwater Observatory, Horsham, West Sussex, UK
⁵⁵ American Association of Variable Stars Observers (AAVSO), Tetoora Road Observatory, 2643 Warragul-Korumburra Road, Tetoora Road 3821, Australia
⁵⁶ International Astronomical Union Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference, NOIRLab, 950 N. Cherry Ave, Tucson, AZ 85719, USA

^⋆ Corresponding author: gag@astro.unam.mx

Received: 12 July 2025
Accepted: 4 September 2025

Abstract

Context. The overwhelming majority of cataclysmic variables (CVs) have orbital periods shorter than 10 h. However, a few have much longer orbital periods, and their formation and existence pose certain challenges for the CV evolution models. These extremely long-period CVs must host nuclearly evolved donor stars (i.e., subgiants), as the companion of the white dwarf would otherwise be too small to fill its Roche lobe. This makes the extremely long-period CVs natural laboratories for testing binary evolution models and accretion processes with subgiant donors, with applications extending beyond white dwarf binaries. Despite the importance of compact objects accreting from subgiant donors, the process by which they form and evolve remains unclear.

Aims. To shed light on the formation and evolution of accreting compact objects with subgiant companions, we investigated two extremely long-period CVs in detail, namely V479 And (P_orb ≃ 14 h) and V1082 Sgr (P_orb ≃ 21 h). We searched for reasonable formation pathways to explain their refined stellar and binary parameters.

Methods. We used a broad set of new observations, including ultraviolet and infrared spectroscopy, results of circular polarimetry, and improved Gaia DR3 distance estimates, to determine the fundamental parameters (e.g., effective temperatures, masses, and radii of the donor stars) that would be confronted with numerical simulations. Furthermore, we utilized the MESA code to conduct numerical simulations, employing state-of-the-art prescriptions, such as the Convection And Rotation Boosted (CARB) model for strong magnetic braking.

Results. The two systems have an unusual chemical composition and very low masses for their assigned spectral classes. This most likely indicates that they underwent thermal timescale mass transfer. We found models for the two extremely long-period CVs that can reasonably reproduce their properties. CV evolution needs to be convergent (i.e., toward shorter orbital periods), which is only possible if the magnetic braking is sufficiently strong.

Conclusions. We conclude that the donor stars in both V479 And and V1082 Sgr are filling their Roche lobes, ruling out previous models in which they are underfilling their Roche lobes. Our findings suggest that orbital angular momentum loss is stronger due to magnetic braking in CVs with subgiant donors compared to those with unevolved donors. In addition, our findings suggest that extremely long-period CVs could significantly contribute to the population of double white dwarf binaries in close orbits (orbital periods ≲1 d).