An eclipsing post common-envelope system consisting of a pulsating hot subdwarf B star and a brown dwarf companion

Abstract

Hot subdwarf B stars (sdBs) are evolved, core helium-burning objects located on the extreme horizontal branch. Their formation history is still puzzling as the sdB progenitors must lose nearly all of their hydrogen envelope during the red-giant phase. About half of the known sdBs are in close binaries with periods from 1.2 h to a few days, a fact that implies they experienced a common-envelope phase. Eclipsing hot subdwarf binaries (also called HW Virginis systems) are rare but important objects for determining fundamental stellar parameters. Even more significant and uncommon are those binaries containing a pulsating sdB, as the mass can be determined independently by asteroseismology. Here we present a first analysis of the eclipsing hot subdwarf binary V2008-1753. The light curve shows a total eclipse, a prominent reflection effect, and low--amplitude pulsations with periods from 150 to 180 s. An analysis of the light-- and radial velocity (RV) curves indicates a mass ratio close to $q=0.146$, an RV semi-amplitude of $K=54.6\mathrm{km}{\mathrm{s}}^{-1}$, and an inclination of $i=86.8^{\circ }$. Combining these results with our spectroscopic determination of the surface gravity, $\log g=5.83$, the best--fitting model yields an sdB mass of 0.47$M_{\odot }$ and a companion mass of $69M_{\mathrm{Jup}}$. As the latter mass is below the hydrogen-burning limit, V2008-1753 represents the first HW Vir system known consisting of a pulsating sdB and a brown dwarf companion. Consequently, it holds great potential for better constraining models of sdB binary evolution and asteroseismology.

9 pages, 8 figures, accepted for A&A

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