1999MNRAS.304..443M

I present and analyse time-resolved B-band spectra of the double-degenerate binary star GP Com. The spectra confirm the presence and period (46.5 min) of the `S'-wave feature found by Nather, Robinson & Stover. GP Com is erratically variable at X-ray and UV wavelengths. I find the equivalent variability in my data, which, as also seen in UV data, is mostly confined to the emission lines. The He{emsp13}ii 4686 changes by the largest amount, consistent with X-ray-driven photoionization. The flaring part of the line profiles is broader than the average, as expected if they are dominated by the inner disc. The He{emsp13}ii 4686 profile is especially remarkable in that its blueshifted peak is 1400 km.s^–1 from line centre compared with 700 km.s^–1 for the He{emsp13}i lines (the redshifted peak is blended with He{emsp13}i 4713). I deduce that He{emsp13}ii 4686 emission is confined to the inner 1/4 of the disc. I also suggest that the activity of the inner disc indicates that accretion is significant (and unstable) there, in contrast to quiescent dwarf novae; this supports models in which GP Com is in a (quasi-)steady state of low mass transfer rate.

GP Com shows triple-peaked line profiles, which consist of the usual double-peaked profiles from a disc plus a narrow component at line centre. The latter has previously been ascribed to emission from a nebula, although none could be found in direct images. However, I find evidence for both radial velocity and flux variability in this component, inconsistent with a nebula origin. The radial velocity amplitude of 10.8±1.6 km.s^–1 and its phase relative to the `S'-wave are consistent with an origin on the accreting white dwarf, if the mass ratio, q=M₂/M₁, is of order 0.02, as expected on evolutionary grounds. However, this explanation is still not satisfactory, as the systemic velocity of the narrow component shows significant variation from line to line, and I have no explanation for this.