THE MASS, TEMPERATURE AND DISTANCE OF THE WHITE-DWARF IN V471 TAURI

We present an analysis of far-UV observations of V471 Tauri made durin g the first ORFEUS mission. Combining this spectrum with archival data from IUE allows a first unambiguous measurement of the effective temp erature and surface gravity of the white dwarf by matching the observe d Lyman line profiles to the predictions of stellar model atmosphere c alculations (T-eff = 32 400 +/- 270/-800 K and log g = 8.16 + 0.18/-0. 24). However, a mass estimate of the white dwarf based on the Bois et al. mass function provides a more precise value for log g (8.27 +/- 0. 07). Using the accurate Hipparcos parallax for the system, from which we obtain a distance of 47 +/- 4 pc, we are able to obtain an independ ent measurement of the white dwarf radius and, therefore, its mass. Co mparison with the theoretical mass-radius relation of Wood, for a star at this temperature, confirms that these evolutionary calculations ar e a good representation of the white dwarf structure. This lends confi dence to spectroscopic mass determinations and estimates of the age of the galactic disc, inferred from the white dwarf luminosity function and cooling ages, based on these models. The white dwarf, now confirme d by the Hipparcos parailax to be a member of the Hyades cluster, is c learly the hottest and possibly the most massive and youngest white dw arf in that cluster. Finally, weak absorption features in the ORFEUS d ata hint at the possible presence of nitrogen and iron in the photosph ere of the white dwarf.