The discovery of high-velocity flares in NV and the detection of carbon in the double degenerate binary GP Com

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Abstract
We present time-resolved ultraviolet spectra of the ultrashort-period binary GP Com taken with the Hubble Space Telescope. During our observations GP Com displayed spectacular variability in the dominant NV emission line which showed three large flares in 13h of observations. We isolate the flare spectrum which reveals CIV emission at a level of 0.094 times the NV flux; this is the first detection of carbon in GP Com. We obtain a 3σ upper limit of 0.015 upon the ratio of Si IV to N V emission, supporting evidence from optical data that heavy elements are under-abundant relative to nitrogen in GP Com. There are many other features in the spectrum that we have been unable to identify. We suggest that the flare emission could come from irradiation of the accretion disc in GP Com or from variable mass loss in a wind. Subtraction of the flare spectrum followed by phase-folding on the 46-min orbital period of GP Com reveals the ultraviolet counterpart to the ‘S’-wave known from optical data. The ‘S’-wave is seen in NV, CIV, NI 1493, and a line at 1335 Å, which may be C II 1335.

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