The hot subdwarf in the eclipsing binary HD 185510

HD 185510 (= V1379 Aql) is an eclipsing binary containing a K0 III/IV star with an active chromosphere and a hot subluminous companion (HD 185510B), previously classified as a subdwarf B star. Radial velocities for the hot star measured at both quadratures with IUE show that the system has a large mass ratio, $$M_\text p/M_\text s = 7.45\pm 0.15$$. Atmospheric parameters derived from UV spectrophotometry and Lyα indicate that T_eff = 25000 ± 1000 K and log g ∼ 6.0. Ground-based and ultraviolet photometry constrain the orbital inclination ($$90^\circ \ge i\ge70^\circ$$), indicating masses of $$2.3-2.8\enspace M_\odot$$ for the K star and $$0.31-0.37\enspace M_\odot$$ for the hot star. The observation bears directly on theories for the late evolution of low-mass stars. HD 185510B is not a subdwarf B star in the classical sense, since its surface gravity is too high. It is probable that HD 185510 is a post-Algol system, in which enhanced mass loss during late case B mass exchange accounts for the low mass of the hot star. HD 185510B is either a low-mass helium main-sequence star or a nascent helium degenerate. It will ultimately become a low-mass ($$\sim 0.3\enspace M_\odot$$) white dwarf.