2005MNRAS.361..487R

We present high time-resolution spectroscopy with the Very Large Telescope of SDSS J124058.03-015919.2, a new helium-transferring binary star identified in the Sloan Digital Sky Survey. We measure an orbital period of 37.355 ± 0.002 min, confirming the AM CVn nature of the system. From the velocity amplitudes of the accretor and the accretion stream-disc impact, we derive a mass ratio q= 0.039±0.010. Our spectral coverage extends from λλ3700-9500 Å and shows the presence of helium, nitrogen, silicon and iron in the accretion disc, plus the redshifted, low-velocity `central spikes' in the helium lines, known from the low-state AM CVn stars GP Com and CE 315. Doppler tomography of the helium and silicon emission lines reveals an unusual pattern of two bright emission sites in the tomograms, instead of the usual one emission site identified with the impact of the mass stream into the accretion disc. One of the two is preferred as the conventional stream-disc impact point in velocity space, at the 3σ confidence level. We speculate briefly on the origin of the second.