  We obtained Hubble GHRS medium resolution (G160M grating) phase-resolved
spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf
nova quiescence, 13 days and 61 days following the end of a narrow outburst.
The spectral wavelength ranges were centered upon three different line regions:
N V (1238\AA, 1242\AA), Si III (1300\AA) and He II (1640\AA). All of the
quiescent spectra at both epochs are dominated by absorption lines and show no
emission features. The Si III and He II absorption line velocities versus
orbital phase trace the orbital motion of the white dwarf but the N~V
absorption velocities appear to deviate from the white dwarf motion. We confirm
our previously reported low white dwarf rotational velocity, V sin i= 100 km/s.
We obtain a white dwarf orbital velocity semi-amplitude K1=107 km/s. Using the
gamma-velocity of Wade (1981) we obtain an Einstein redshift of 80.4 km/s and
hence a carbon core white dwarf mass of ~1.1 Msun. We report the first subsolar
chemical abundances of C and Si for U Gem with C down by 0.05 with respect to
the Sun, almost certainly a result of C depletion due to thermonuclear
processing. This C-depletion is discussed within the framework of a weak TNR,
contamination of the secondary during the common envelope phase, and mixing of
C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova
event. Remarkably the Teff of the white dwarf 13 days after outburst is only
32,000K, anomalously cooler than previous early post-outburst measurements.
Extensive cooling during an extraordinarily long (210 days) quiescence followed
by accretion onto an out-of-equilibrium cooled degenerate could explain the
lower Teff.

Astrophysics