Hot donors in cataclysmic variables: The case of EI Psc

¹ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, CZ-180 00 Praha 8, Czech Republic
² Department of Physics and Astronomy, University of Texas Rio Grande Valley, Brownsville, TX 78520, USA
³ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada, 22800 BC, Mexico
⁴ Faculty of Physics and Technology, Al-Farabi Kazakh National University, Al-Farabi Ave., 71, Almaty 050040, Kazakhstan
⁵ Fesenkov Astrophysical Institute, 23 Observatory Str., Almaty 050020, Kazakhstan
⁶ Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Monterrey, Nuevo León 66451, Mexico

^⋆ Corresponding author: honza.kara.7@gmail.com

Received: 30 January 2025
Accepted: 22 May 2025

Abstract

Context. We present results of time-resolved optical spectroscopy and photometry of the short-orbital period dwarf nova EI Psc.

Aims. This study aims to determine fundamental system parameters of EI Psc, study properties of accretion structures in the system, and investigate its origin and current evolution state.

Methods. We analyse newly obtained time-resolved spectroscopic and photometric observations as well as archival data. We used light curve modelling, Doppler tomography, and MESA evolutionary models to study the characteristics of EI Psc.

Results. The system contains a relatively low temperature (T_eff = 6130 K) white dwarf with mass of M_WD = 0.70(4) M_⊙. The mass of the warm (T₂ = 4440 K) secondary is M₂ = 0.13 M_⊙. The inclination of the system is i = 44_.^°5(7). The mass accretion rate is ≈4 × 10⁻¹³ M_⊙ year⁻¹. The long-term light curve of the system shows outbursts and superoutbursts. The quiescence light curve is double-humped and is formed by the combination of radiation from the Roche lobe filling the hot secondary and the hot spot. The radius of the outer disc is about two times smaller than the tidal truncation radius. Most of the disc’s emission consists of emission lines and radiation from the hot spot at the stream-disc impact region.

Conclusions. These types of systems are formed from progenitors with a low mass WD M_WD ≲ 0.6 M_⊙ and relatively massive secondaries 1.1 − 1.5 M_⊙ with initial orbital periods on a scale of days. The number of similar systems is expected to be significantly lower than the usual CVs due to a lower forming rate of their relatively massive progenitors.