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.
@article{arxiv.1204.1338,
title = {SDSS J184037.78+642312.3: The First Pulsating Extremely Low Mass White Dwarf},
author = {J. J. Hermes and M. H. Montgomery and D. E. Winget and Warren R. Brown and Mukremin Kilic and Scott J. Kenyon},
journal= {arXiv preprint arXiv:1204.1338},
year = {2015}
}