Upper panel: mass distribution of all DO WDs from our sample. Middle panel: mass distribution of PG 1159 stars (thin black line), O(He) stars and DO WDs before the wind limit (thick red line), and DO WDs beyond the wind limit (gray area, light pink in the online version). Lower panel: mass distribution of the hot (T_eff  ≥ 45 kK) DA WDs (gray area, light blue in the online version) from the sample of Gianninas et al. (2011) compared with the mass distribution of non-DA WDs (PG 1159 and DO WDs with log g  ≥ 7, thick red line).