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 { T } _ { \mathrm { eff } } = 9100 \pm 170 K and \log { g } = 6.22 \pm 0.06 , which corresponds to a mass \sim 0.17 M _ { \odot } .
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 .