The hot subdwarf in the eclipsing binary HD185510

C.S.Jeffery and T.Simon

The importance of binarity in the origin and evolution of hot subdwarfs has become widely recognised. The RS CVn binary HD185510 consists of a K0 III giant which eclipses its faint blue companion every 21 days. The latter has been identified as an sdB star: it is the only such object known in a wide eclipsing binary and can only be observed in the ultraviolet, where its radial velocity amplitude and spectrum have been recorded. A detailed analysis of the orbit from a new ultraviolet light curve and previous radial velocities indicates a low mass (~0.3 solar masses) for the blue star. We have derived atmospheric parameters from multiwavelength photometry and from high-resolution ultraviolet spectra. Both components appear to be metal poor, the hot star is intermediate between a classical subdwarf B star and a helium white dwarf, with Teff~31000 K. In order to resolve a conflict between the primary radius indicated by the rotational light curve and that indicated by the eclipse geometry, it is necessary to assume that the incliniation i~90 and that the subdwarf is partially eclipsed by the atmosphere of the K giant with an optical scale height of ~0.03 stellar radii. The evolutionary implications are considered.