SDSS J184037.78+642312.3: The First Pulsating Extremely Low Mass White
  Dwarf
  We report the discovery of the first pulsating extremely low mass (ELM) white
dwarf (WD), SDSS J184037.78+642312.3 (hereafter J1840). This DA
(hydrogen-atmosphere) WD is by far the coolest and the lowest-mass pulsating
WD, with Teff = 9100 \pm 170 K and log g = 6.22 \pm 0.06, which corresponds to
a mass ~ 0.17 Msun. This low-mass pulsating WD greatly extends the DAV (or ZZ
Ceti) instability strip, effectively bridging the log g gap between WDs and
main sequence stars. We detect high-amplitude variability in J1840 on
timescales exceeding 4000 s, with a non-sinusoidal pulse shape. Our
observations also suggest that the variability is multi-periodic. The star is
in a 4.6 hr binary with another compact object, most likely another WD. Future,
more extensive time-series photometry of this ELM WD offers the first
opportunity to probe the interior of a low-mass, presumably He-core WD using
the tools of asteroseismology.
