With typical periods of order 10 minutes , the pulsation signatures of ZZ Ceti variables ( pulsating hydrogen-atmosphere white dwarf stars ) are severely undersampled by long-cadence ( 29.42 minutes per exposure ) K2 observations .
Nyquist aliasing renders the intrinsic frequencies ambiguous , stifling precision asteroseismology .
We report the discovery of two new ZZ Cetis in long-cadence K2 data : EPIC 210377280 and EPIC 220274129 .
Guided by 3–4 nights of follow-up , high-speed ( \leq 30 s ) photometry from McDonald Observatory , we recover accurate pulsation frequencies for K2 signals that reflected 4–5 times off the Nyquist with the full precision of over 70 days of monitoring ( \sim 0.01 \mu Hz ) .
In turn , the K2 observations enable us to select the correct peaks from the alias structure of the ground-based signals caused by gaps in the observations .
We identify at least seven independent pulsation modes in the light curves of each of these stars .
For EPIC 220274129 , we detect three complete sets of rotationally split \ell = 1 ( dipole mode ) triplets , which we use to asteroseismically infer the stellar rotation period of 12.7 \pm 1.3 hr .
We also detect two sub-Nyquist K2 signals that are likely combination ( difference ) frequencies .
We attribute our inability to match some of the K2 signals to the ground-based data to changes in pulsation amplitudes between epochs of observation .
Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC 220274129 near the middle of the ZZ Ceti instability strip , with { T } _ { \mathrm { eff } } = 11590 \pm 200 K and 11810 \pm 210 K , and masses 0.57 \pm 0.03 M _ { \odot } and 0.62 \pm 0.03 M _ { \odot } , respectively .