  U Gem is unique in having a direct measurement of K1 = 107 +- 2 km/s, Long et
al. 1999). We present high-resolution optical spectra of the dwarf nova U Gem
in quiescence taken to test the accuracy to which the HST value can be
recovered from optical data. We find that, even with data of very high S/N we
cannot recover Long et al.'s value to better than about 20% by any method.
Contamination by neighbouring emission lines seems a likely culprit. Our data
reveal a number of new features: Doppler tomograms show emission at low
velocity, close to the centre of mass, and a transient, narrow absorption
feature is seen in the Balmer lines near the line centres at the time of
eclipse. We suggest that stellar prominences, as previously invoked for the
dwarf novae IP Peg and SS Cyg in outburst, may explain both of these features.
The He II 4686.75 A line emission is dominated by the gas stream/disc impact
region. Two distinct spots are seen in Doppler maps, the first being very
narrow and showing a velocity close to that of the accretion disc in the impact
region, and the second much broader and located between the velocities of the
(ballistic) stream and the (Keplerian) disc. We present tentative evidence of
weak spiral structure. We find no evidence of stream-disc overflow in the
system. Our data suggests an inclination angle > 70 degrees, adding to the
evidence of a puzzle in the mass of U Gem's white dwarf. The mass donor is
clearly seen in the Doppler maps, with emission concentrated towards its poles,
and mainly on the side facing the white dwarf. This suggests irradiation with
shielding by the disc from which we estimate an H/R ratio between 0.15 and
0.25.
