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We report the discovery of the second and third pulsating extremely low masswhite dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSSJ151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 Msun andeffective temperatures below 10,000 K, establishing these putatively He-coreWDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Cetistars). The short-period pulsations evidenced in the light curve of J1112 mayalso represent the first observation of acoustic (p-mode) pulsations in any WD,which provide an exciting opportunity to probe this WD in a complimentary waycompared to the long-period g-modes also present. J1112 is a Teff = 9590 +/-140 K and log(g) = 6.36 +/- 0.06 WD. The star displays sinusoidal variabilityat five distinct periodicities between 1792-2855 s. In this star we also seeshort-period variability, strongest at 134.3 s, well short of expected g-modesfor such a low-mass WD. The other new pulsating WD, J1518, is a Teff = 9900 +/-140 K and log(g) = 6.80 +/- 0.05 WD. The light curve of J1518 is highlynon-sinusoidal, with at least seven significant periods between 1335-3848 s.Consistent with the expectation that ELM WDs must be formed in binaries, thesetwo new pulsating He-core WDs, in addition to the prototype SDSSJ184037.78+642312.3, have close companions. However, the observed variabilityis inconsistent with tidally induced pulsations and is so far best explained bythe same hydrogen partial-ionization driving mechanism at work in classicC/O-core ZZ Ceti stars.

Discovery of pulsations, including possible pressure modes, in two new extremely low mass, He-core white dwarfs