HS 2231+2441: an HW Vir system composed of a low-mass white dwarf and a brown dwarf★

HW Vir systems are rare evolved eclipsing binaries composed of a hot compact star and a low-mass main sequence star in a close orbit. These systems provide a direct way to measure the fundamental properties, e.g. masses and radii, of their components, hence they are crucial in studying the formation of subdwarf B stars and low-mass white dwarfs, the common-envelope phase and the pre-phase of cataclysmic variables. Here, we present a detailed study of HS 2231+2441, an HW Vir type system, by analysing BVR_CI_C photometry and phase-resolved optical spectroscopy. The spectra of this system, which are dominated by the primary component features, were fitted using non-local thermodynamic equilibrium models providing an effective temperature T_eff = 28 500 ± 500 K, surface gravity log g = 5.40 ± 0.05 cm s^-2 and helium abundance log (n(He)/n(H)) = -2.52 ± 0.07. The geometrical orbit and physical parameters were derived by simultaneously modelling the photometric and spectroscopic data using the Wilson-Devinney code. We derive two possible solutions for HS 2231+2441 that provide the component masses: M₁ = 0.19 M_⊙ and M₂ = 0.036 M_⊙ or M₁ = 0.288 M_⊙ and M₂ = 0.046 M_⊙. Considering the possible evolutionary channels for forming a compact hot star, the primary of HS 2231+2441 probably evolved through the red-giant branch scenario and does not have a helium-burning core, which is consistent with a low-mass white dwarf. Both solutions are consistent with a brown dwarf as the secondary.