THE COMPLETE SPECTRAL ENERGY-DISTRIBUTION AND THE ATMOSPHERIC PROPERTIES OF THE HELIUM-RICH WHITE-DWARF MCT 0501-2858

We analyze high-dispersion ultraviolet spectroscopic observations of t he peculiar helium-rich white dwarf MCT 0501 - 2858. Ultraviolet spect ra obtained during the 1996 November ORFEUS-SPAS II mission show the c omplete He II (n = 2) line series as well as narrow heavy-element line s; the spectrum constrains the effective temperature (68,600 +/- 1800 K) and surface gravity (log g = 7.20 +/- 0.07), imposing a low mass (0 .4 Mo) and short cooling age (10(5) yr) for the star A study of the co mplete spectral energy distribution, based on EUVE, ORFEUS, and IUE sp ectra, shows that carbon is the second most abundant element [log (C/H e) = -1.9] and that lower abundances are measured for other elements [ log (N/He) = -4.0, log (O/He) = -3.3, log (Si/He) = -4.6, and log (P/H e) = -6.2]. The new atmospheric parameters (temperature, gravity, and chemical composition) suggest a possible evolutionary filiation with t he class of PG 1159 stars.