Timing the eclipse of HD185510

C.S.Jeffery and T.Simon

The atmospheric parameters of both components have been investigated. From its out-of-eclipse flux distribution, the K0 star has Teff=4500 K and appears to be metal-deficient. However these measurements are sensitive to the fractional spot coverage at the time photometry was obtained.

The IUE flux distribution, low-resolution Ly-alpha profile and a noisy high-resolution UV spectrum of the hot companion have been analysed to obtain Teff=31500+/-1500 K, log g=7.2+/-0.3 and E(B-V)=0.13+/-0.03. C and Si are deficient by ~1 dex; it is not possible to deduce other metal abundances from the existing data. The spectroscopic gravity is higher than indicated by the eclipse geometry, indicating a weakness in the Ly-alpha analysis. Neglecting Ly-alpha, an alternative solution of Teff=31000+/-1500 K, log g=6.5+/-0.2, and E(B-V)=0.1+/-0.03 is obtained from the orbital solution below. The relative diameters of the subdwarf and K giant are r_s/r_p=0.0058+/-0.0010.

These data were used with the eclipse geometry in an attempt to obtain the orbital inclination. In order to avoid severe contradictions with other diagnostics it was necessary to introduce a non-negligible eclipse due to the cool star atmosphere. Whilst available data favour i=90+/-5, uncertainties introduced by the atmospheric eclipse mean that i~80 is also possible.

With i=90, the masses of the cool and hot stars are 2.27+/-0.17 and 0.304+/-0.015 M_sun respectively. Whilst the high-gravity (Ly-alpha) solution indicates the identification of the hot star as a helium white dwarf, the orbital solution favours an identification with sdOB stars such as SB707. A final resolution of the nature of the hot star is limited partly by data quality but also, to a large extent, by the intrinsic properties of the system itself.