  We have detected coherent oscillations (``dwarf nova oscillations'') in
Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were
seen towards the end of a superoutburst of OY Car. The oscillations are
extraordinary compared to the many other examples in the literature for two
reasons. First, their amplitude is large, with a peak-to-peak variation of 8 to
20% of the total flux over the range 1100 to 2500A. However, most remarkably we
find that there are two components present simultaneously. Both have periods
close to 18sec (equivalent to 4800 cycles/day) but they are separated by
57.7+/-0.5 cycles/day. The lower frequency component of the pair has a strong
second harmonic while its companion, which has about twice its amplitude, does
not. The oscillation spectra appear hotter than the mean spectrum and
approximately follow the continuum distribution of a black-body with a
temperature in the range 30,000 to 50,000K.
  We tentatively suggest that the weaker non-sinusoidal component could
represent the rotation of the white dwarf, although we have been unable to
recover any such signal in quiescent data.
