We present the serendipitous discovery of eclipse-like events around the
massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We
selected J1529+2928 for time-series photometry based on its spectroscopic
temperature and surface gravity, which place it near the ZZ Ceti instability
strip. Instead of pulsations, we detect photometric dips from this white dwarf
every 38 minutes. Follow-up optical spectroscopy observations with Gemini
reveal no significant radial velocity variations, ruling out stellar and brown
dwarf companions. A disintegrating planet around this white dwarf cannot
explain the observed light curves in different filters. Given the short period,
the source of the photometric dips must be a dark spot that comes into view
every 38 min due to the rotation of the white dwarf. Our optical spectroscopy
does not show any evidence of Zeeman splitting of the Balmer lines, limiting
the magnetic field strength to B<70 kG. Since up to 15% of white dwarfs display
kG magnetic fields, such eclipse-like events should be common around white
dwarfs. We discuss the potential implications of this discovery on transient
surveys targeting white dwarfs, like the K2 mission and the Large Synoptic
Survey Telescope.Comment: ApJ Letters, in pres
