  We describe a spectroscopic survey designed to uncover an estimated ~40 AM
CVn stars hiding in the photometric database of the Sloan Digital Sky Survey
(SDSS). We have constructed a relatively small sample of about 1500 candidates
based on a colour selection, which should contain the majority of all AM CVn
binaries while remaining small enough that spectroscopic identification of the
full sample is feasible.
  We present the first new AM CVn star discovered using this strategy, SDSS
J080449.49+161624.8, the ultracompact binary nature of which is demonstrated
using high-time-resolution spectroscopy obtained at the Magellan telescopes at
Las Campanas Observatory, Chile. A kinematic 'S-wave' feature is observed on a
period 44.5+/-0.1min, which we propose is the orbital period, although the
present data cannot yet exclude its nearest daily aliases.
  The new AM CVn star shows a peculiar spectrum of broad, single-peaked helium
emission lines with unusually strong series of ionised helium, reminiscent of
the (intermediate) polars among the hydrogen-rich Cataclysmic Variables. We
speculate that SDSS J0804+1616 may be the first magnetic AM CVn star. The
accreted material appears to be enriched in nitrogen, to N/O>~10 and N/C>10 by
number, indicating CNO-cycle hydrogen burning, but no helium burning, in the
prior evolution of the donor star.

Astrophysics