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 cont inuum and emission lines appear reduced by factors of, respectively, s imilar or equal to 4 and approximate to 5. Flickering activity is most ly 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 geomet ry and to estimate masses and relevant dimensions. We find a mass rati o of q = 0.19 +/- 0.03 and an inclination of i = 83 degrees +/- 2 degr ees. 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 m ade. 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.