THE HOT SUBDWARF IN THE ECLIPSING BINARY HD185510
Institute for Astronomy, University of Hawaii
ABSTRACT: 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 highresolution 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 T eff ¸ 31 000K. 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 ffi and that the
subdwarf is partially eclipsed by the atmosphere of the K giant with an optical scale height
of ¸ 0:03R ? . The evolutionary implications are considered.