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The Hopkins Ultraviolet Telescope on Astro--2 was used to obtain a FUV (830 -- 1860 \AA) spectrum of the dwarf nova U Geminorum in the low state. At the time of the observation, U Gem had been at optical minimum for 185 days; an outburst of the system occurred 24 days later, exceeding the previous longest observed interoutburst period for the system. The HUT spectrum is dominated by the white dwarf star, and shows (among other things) very strong, broad hydrogen Lyman absorption lines. A comparison with white dwarf models indicates a temperature of $\sim$30,000 K is appropriate, assuming a single temperature model is appropriate. If heating takes place differentially on the white dwarf's surface, as may well occur, this represents an ``average'' temperature.
U Gem was also observed during the Astro--1 mission in 1990, 10 days after U Gem had returned to the low state following a normal outburst. The spectrum shows many differences from that obtained during Astro--2. In particular, the Lyman lines are much weaker and the ``average'' temperature of the white dwarf surface was $\sim$38,000 K. The HUT temperature determinations confirm that the white dwarf in U Gem cools substantially between outbursts. We will compare these two spectra in detail, and discuss the possibility that the temperature distribution on the surface of the white dwarf is non-uniform. Other contributors to the FUV emission in the system will be discussed.
This work was supported by NASA contract NAS 5-27000 to the Johns Hopkins University.
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