Evolutionary tracks (red solid lines) of ultra-massive DA WD models computed by Camisassa et al. (2019) in the T_eff − log g plane. Blue dashed lines indicate 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95 and 99% of crystallised mass. Locations of ultra-massive DA WD stars (Kleinman et al. 2013; Kepler et al. 2016; Curd et al. 2017) are indicated with black star symbols. Black circles indicate the location of known ultra-massive ZZ Ceti stars: BPM 37093 (Nitta et al. 2016), SDSS J084021 (Curd et al. 2017), GD 518 (Hermes et al. 2013), and WD J212402 (Rowan et al. 2019).