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We present the first comprehensive spectroscopic overview of a hot degenerate star with observations of MCT 0501-2858 (=EUVE 0503-289), covering the complete energy distribution from 70 to 6700 \AA\ . Optical spectroscopy reveals a helium-rich atmosphere with dominant He II and weak He I line absorption characteristic of the class of cool DO white dwarfs defined by Wesemael, Green, \& Liebert (1985, ApJS, 58, 379), but also shows the C IV/He II blend near 4650--4690 \AA\ characteristic of ultra-hot PG 1159 stars like MCT 0130-1937 with effective temperatures $> 10^5 K$. However, Extreme Ultraviolet Explorer (EUVE ) photometric measurements have already imposed a much lower temperature of $T_{eff} = 60-70,000 K$ (Vennes et al. 1994, ApJ, 421, L35). MCT 0501-2858 is very peculiar and has spectroscopic properties intermediate to the cool DO white dwarfs and their hot PG 1159 progenitors.\\
New extreme ultraviolet (EUV) spectroscopy obtained with EUVE reveals a bright continuum overlaid by rich He II, C III and C IV line spectra --truncated below $\sim 230$ \AA\ by He II photoionization. The spectrum is almost extinct near 750 \AA\ due to interstellar attenuation; we derive a neutral hydrogen column density of $6-10 \times 10^{18} cm^{-2}$, consistent with earlier photometric estimates. Using new Opacity Project radiative cross-sections and our comprehensive optical/EUV C IV line absorption spectrum--in particular the lines emerging from 2s, 2p, 3s, 5p, 5d--we determine a carbon abundance 3 to 10 times larger than the solar abundance, while other heavy elements are only marginally present. This extraordinary carbon abundance is reminiscent of the peculiar C-rich atmosphere of hot PG 1159 stars and may be the result of on-going chemical separation in photospheric layers. This work has been supported by NASA grant NAG5-2405.
