HST and ground - based eclipse observations of V 2051 Ophiuchi : Binary parameters ⋆

We report on high-speed eclipse photometry of the dwarf nova V2051 Oph while it was in a low brightness state, at B ≃ 16.2 mag. In comparison to the average IUE spectra, the ultraviolet continuum and emission lines appear reduced by factors of, respectively, ≃ 4 and ≃ 5. Flickering activity is mostly suppressed and the lightcurve shows the eclipse of a compact white dwarf at disc centre which contributes ≃ 60 per cent of the total light at 3900–4300 Å. We use measurements of contact phases in the eclipse lightcurve to derive the binary geometry and to estimate masses and relevant dimensions. We find a mass ratio of q = 0.19 ± 0.03 and an inclination of i = 83 ± 2 degrees. The masses of the component stars are M1 = 0.78±0.06 M⊙ and M2 = 0.15±0.03 M⊙. Our photometric model predicts K1 = 83± 12 km s −1 and K2 = 435± 11 km s . The predicted value of K1 is in accordance with the velocity amplitude obtained from the emission lines after a correction for asymmetric line emission in the disc is made (Watts et al. 1986). The secondary of V2051 Oph is significantly more massive than the secondaries of the other ultra-short period dwarf novae. V2051 Oph is probably a relatively young system, whose secondary star had not enough time to evolve out of thermal equilibrium.