2017MNRAS.468..239C

We report the discovery of four massive (M > 0.8 M_☉) ZZ Ceti white dwarfs, including an ultramassive 1.16 M_☉ star. We obtained ground-based, time series photometry for 13 white dwarfs from the Sloan Digital Sky Survey Data Release 7 and Data Release 10 whose atmospheric parameters place them within the ZZ Ceti instability strip. We detect monoperiodic pulsations in three of our targets (J1015, J1554 and J2038) and identify three periods of pulsation in J0840 (173, 327 and 797 s). Fourier analysis of the remaining nine objects does not indicate variability above the 4<A> detection threshold. Our preliminary asteroseismic analysis of J0840 yields a stellar mass M = 1.14 ± 0.01 M_☉, hydrogen and helium envelope masses of M_H = 5.8 x 10^–7 M_☉ and M_He = 4.5 x 10^–4 M_☉ and an expected core crystallized mass ratio of 50-70 per cent. J1015, J1554 and J2038 have masses in the range 0.84-0.91 M_☉ and are expected to have a CO core; however, the core of J0840 could consist of highly crystallized CO or ONeMg given its high mass. These newly discovered massive pulsators represent a significant increase in the number of known ZZ Ceti white dwarfs with mass M > 0.85 M_☉, and detailed asteroseismic modelling of J0840 will allow for significant tests of crystallization theory in CO and ONeMg core white dwarfs.