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We report the discovery of pulsations in the extremely low mass (ELM), likelyhelium-core white dwarf GD 278 via ground- and space-based photometry. GD 278was observed by the Transiting Exoplanet Survey Satellite (TESS) in Sector 18at a 2-min cadence for roughly 24 d. The TESS data reveal at least 19significant periodicities between 2447-6729 s, one of which is the longestpulsation period ever detected in a white dwarf. Previous spectroscopy foundthat this white dwarf is in a 4.61 hr orbit with an unseen >0.4 solar-masscompanion and has Teff = 9230 +/- 100 K and log(g) = 6.627 +/- 0.056, whichcorresponds to a mass of 0.191 +/- 0.013 solar mass. Patterns in the TESSpulsation frequencies from rotational splittings appear to reveal a stellarrotation period of roughly 10 hr, making GD 278 the first ELM white dwarf witha measured rotation rate. The patterns inform our mode identification forasteroseismic fits, which unfortunately do not reveal a global best-fitsolution. Asteroseismology reveals two main solutions roughly consistent withthe spectroscopic parameters of this ELM white dwarf, but with vastly differenthydrogen-layer masses; future seismic fits could be further improved by usingthe stellar parallax. GD 278 is now the tenth known pulsating ELM white dwarf;it is only the fifth known to be in a short-period binary, but is the firstwith extended, space-based photometry.

Discovery, TESS Characterization, and Modeling of Pulsations in the Extremely Low Mass White Dwarf GD 278