  We present three new candidate AM CVn binaries, plus one confirmed new
system, from a spectroscopic survey of color-selected objects from the Sloan
Digital Sky Survey. All four systems were found from their helium emission
lines in low-resolution spectra taken on the Hale telescope at Palomar, and the
Nordic Optical Telescope and the William Herschel Telescope on La Palma.
  The ultra-compact binary nature of SDSS J090221.35+381941.9 was confirmed
using phase-resolved spectroscopy at the Keck-I telescope. From the
characteristic radial velocity `S-wave' observed in the helium emission lines
we measure an orbital period of 48.31 +/- 0.08 min. The continuum emission can
be described with a blackbody or a helium white dwarf atmosphere of T_eff ~
15,000K, in agreement with theoretical cooling models for relatively massive
accretors and/or donors. The absence in the spectrum of broad helium absorption
lines from the accreting white dwarf suggests that the accreting white dwarf
cannot be much hotter than 15,000K, or that an additional component such as the
accretion disk contributes substantially to the optical flux.
  Two of the candidate systems, SDSS J152509.57+360054.5 and SDSS
J172102.48+273301.2, do show helium absorption in the blue part of their
spectra in addition to the characteristic helium emission lines. This, in
combination with the high effective temperatures of ~18,000K and ~16,000K
suggests both two be at orbital periods below ~40min. The third candidate, SDSS
J164228.06+193410.0, exhibits remarkably strong helium emission on top of a
relatively cool (T_eff~12,000K) continuum, indicating an orbital period above
~50min.
