  We present high time resolution VLT spectroscopy 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 minutes, 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 3700A--9500A 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 3-sigma
confidence level. We speculate briefly on the origin of the second.
