TIMING THE ECLIPSE OF HD-185510

HD185510 (=V1379 Aql) is an eclipsing double-lined RS CVn binary conta ining a K0 III/IV giant and a hot subluminous companion. An Internatio nal Ultraviolet Explorer (IUE) ultraviolet light curve has been obtain ed through the eclipse of the companion with an average time resolutio n of one measurement every 19 min. 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 T-eff = 4500+/- 300 K and appears to be metal-deficient. However, these measurements a re sensitive to the fractional spot coverage at the time photometry wa s obtained. The IUE flux distribution, low-resolution Ly alpha profile and a noisy high-resolution UV spectrum of the hot companion have bee n analysed to obtain T-eff = 31500 +/- 1500 K, log g = 7.2 +/- 0.3 and E(B - V) = 0.13 +/- 0.03. C and Si are deficient by similar to 1 dex; it is not possible to deduce other metal abundances from the existing data. The spectroscopic gravity is higher than indicated by the eclip se geometry, indicating a weakness in the Ly alpha analysis, Neglectin g Ly alpha, an alternative solution of T-eff = 31 000 +/- 1500 K, log g = 6.5 +/- 0.2 and E(B - V) = 0.1 +/- 0.03 is obtained from the orbit al solution below. The relative radii of the subdwarf and K giant are related by the secondary-to-primary ratio r(s)/r(p) = 0.0058 +/- 0.001 0. These data were used with the eclipse geometry in an attempt to obt ain the orbital inclination. In order to avoid severe contradictions w ith other diagnostics it was necessary to introduce a non-negligible e clipse due to the cool star atmosphere. Whilst available data favour i = 90 degrees +/- 5 degrees, uncertainties introduced by the atmospher ic eclipse meant that i similar to 80 degrees is also possible. With i = 90 degrees, the masses of the cool and hot stars are 2.27 +/- 0.17 and 0.304 +/- 0.015 M, respectively. Whilst the high-gravity (Ly alpha ) solution indicates the identification of the hot star as a helium wh ite dwarf, the orbital solution favours an identification with sdOB st ars such as SE 707. A final resolution of the nature of the hot star i s limited partly by data quality but also, to a large extent, by the i ntrinsic properties of the system itself.