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HW Vir systems are rare evolved eclipsing binaries composed by a hot compactstar and a low-mass main-sequence star in a close orbit. These systems providea direct way to measure the fundamental properties, e.g. masses and radii, oftheir components, hence they are crucial to study the formation of sdB starsand low-mass white dwarfs, the common-envelope phase, and the pre-phase ofcataclysmic variables. Here we present a detailed study of HS 2231+2441, an HWVir type system, by analysing BVR$_C$I$_C$ photometry and phase-resolvedoptical spectroscopy. The spectra of this system, which are dominated by theprimary component features, were fitted using NLTE models providing effectivetemperature Teff = 28500$\pm$500 K, surface gravity log g = 5.40$\pm$0.05 cms$^{-2}$, and helium abundance log(n(He)/n(H)) = -2.52$\pm$0.07. Geometricalorbit and physical parameters were derived by modelling simultaneously thephotometric and spectroscopic data using the Wilson-Devinney code. We derivetwo possible solutions for HS 2231+2441 that provide component's masses: M$_1$= 0.19 M$_{\odot}$ and M$_2$ = 0.036 M$_{\odot}$ or M$_1$ = 0.288 M$_{\odot}$and M$_2$ = 0.046 M$_{\odot}$. Considering the possible evolutionary channelsto form a compact hot star, the primary of HS 2231+2441 probably evolvedthrough 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 consistentwith a brown dwarf as the secondary.

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