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.