Kinematics of the helium accretor GP Com

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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 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 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 i lines (the redshifted peak is blended with He i 4713). I deduce that He 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.