  The Sloan Digital Sky Survey has been instrumental in obtaining a homogeneous
sample of the rare AM CVn stars: mass-transferring binary white dwarfs. As part
of a campaign of spectroscopic follow-up on candidate AM CVn stars from the
Sloan Digital Sky Survey, we have obtained time-resolved spectra of the g=20.2
candidate SDSS J155252.48+320150.9 on the Very Large Telescope of the European
Southern Observatory. We report an orbital period of 3376.3+/-0.3 s, or
56.272+/-0.005 min, based on an observed `S-wave' in the helium emission lines
of the spectra. This confirms the ultracompact nature of the binary. Despite
its relative closeness to the orbital period minimum for hydrogen-rich donors,
there is no evidence for hydrogen in the spectra. We thus classify SDSS J1552
as a new bona fide AM CVn star, with the second-longest orbital period after
V396 Hya (P=65.5 min). The continuum of SDSS J1552 is compatible with either a
blackbody or helium atmosphere of 12,000-15,000 K. If this represents the
photosphere of the accreting white dwarf, as is expected, it puts the accretor
at the upper end of the temperature range predicted by thermal evolution
models. This suggests that SDSS J1552 consists of (or formerly consisted of)
relatively high-mass components.
