HST and ground-based eclipse observations of V2051 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 similar or equal to 16.2 mag. In comparison with the average IUE spectra, the ultraviolet continuum and emission lines appear reduced by factors of, respectively, similar or equal to 4 and approximate to 5. Flickering activity is mostly suppressed and the light curve shows the eclipse of a compact white dwarf at the disc centre which contributes similar or equal to 60 per cent of the total light at 3900-4300 Angstrom. We use measurements of contact phases in the eclipse light curve 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 degrees +/- 2 degrees. The masses of the component stars are M-1 = 0.78 +/- 0.06 M. and M-2 = 0.15 +/- 0.03 M.. Our photometric model predicts K-1 = 83 +/- 12 km s(-1) and K-2 = 436 +/- 11 km s(-1). The predicted value of K-1 is in accordance with the velocity amplitude obtained from the emission lines after a correction for asymmetric line emission in the disc is made. The secondary of V2051 Oph is significantly more massive than the secondaries of the other ultrashort period dwarf novae. V2051 Oph is probably a relatively young system, with a secondary star that has not had enough time to evolve out of thermal equilibrium.