Mapping the Peculiar Binary Gp Com

We present high resolution spectra of the AM CVn helium binary GP Com at two different wavelength ranges. The spectra show the same flaring behaviour observed in previous UV and optical data. We find that the central spike contributes to the flare spectra indicating that its origin is probably the compact object. We also detect that the central spike moves with orbital phase following an S-wave pattern. The radial velocity semiamplitude of the S-wave is ∼10 km s −1 which indicates its origin is near the centre of mass of the system, which in this case lies very close to the white dwarf. The Stark effect seems to affect significantly the central spike of some of the lines suggesting that it forms in a high electron density region. This again favours the idea that the central spike originates in the white dwarf. We present Doppler maps obtained for the emission lines which show three clear emission regions. GP Com belongs to a group of binaries called AM CVn systems. In these binaries, a white dwarf accretes material from the stripped-down core of a giant star. Only 6 known systems belong to this group although they are predicted to have a space density about a factor of 2 higher than that of cataclysmic variables (CVs) of which some 700 are known [14]. They show properties similar to those of CVs but with some peculiarities, for instance ultra short orbital periods – between 15 and 46 min – which indicate that the two stars are very close together, and a complete lack of hydrogen in their spectra. Some of the systems show strong flickering indicating the presence of mass transfer. AM CVn and EC 15330 show only absorption lines. These lines have large breadths indicating either rapid rotation or pressure broadening by compact stars. CR Boo, CP Eri and V803 Cen show absorption lines when they are bright and emission lines when faint. This absorption/emission line behaviour has been compared to that of CVs, as most of them show emission lines when in quiescence and absorption lines when in outburst. GP Com only shows emission lines and therefore could be considered as always being in quiescence. For a summary of the properties of AM CVn systems see [15]. We took spectra of GP Com in two wavelength ranges, λλ6600 – 7408Å and λλ4253 – 5058Å with the 4.2 m William Herschel telescope …