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Timing the eclipse of HD185510

C.S.Jeffery and T.Simon

HD185510 (=V1379 Aql) is an eclipsing double-lined RS CVn binary containing a K0 III/IV giant and a hot subluminous companion. An IUE ultraviolet light curve has been obtained through the eclipse of the companion with an average time resolution of one measurement every 19 minutes. This was sufficient to resolve all four times of contact and, with data from previous work on this system, to derive a solution for the absolute dimensions of the system.

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.