We report the discovery of the second and third pulsating extremely low mass white dwarfs ( WDs ) , SDSS J111215.82+111745.0 ( hereafter J1112 ) and SDSS J151826.68+065813.2 ( hereafter J1518 ) .
Both have masses < 0.25 M _ { \odot } and effective temperatures below 10 , 000 K , establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs ( DAVs , or ZZ Ceti stars ) .
The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic ( p -mode ) pulsations in any WD , which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g -modes also present .
J1112 is a { T } _ { \mathrm { eff } } = 9590 \pm 140 K and \log { g } = 6.36 \pm 0.06 WD .
The star displays sinusoidal variability at five distinct periodicities between 1792 - 2855 s. In this star we also see short-period variability , strongest at 134.3 s , well short of expected g -modes for such a low-mass WD .
The other new pulsating WD , J1518 , is a { T } _ { \mathrm { eff } } = 9900 \pm 140 K and \log { g } = 6.80 \pm 0.05 WD .
The light curve of J1518 is highly non-sinusoidal , with at least seven significant periods between 1335 - 3848 s. Consistent with the expectation that ELM WDs must be formed in binaries , these two new pulsating He-core WDs , in addition to the prototype SDSS J184037.78+642312.3 , have close companions .
However , the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars .