Location of our three program stars and the comparison star RE 0503−289 (red symbols) together with related objects (Hügelmeyer et al. 2006; Kepler et al. 2016; Reindl et al. 2014a; Werner & Herwig 2006) in the T_eff − g plane. The single triangle symbol denotes a helium-dominated object of the O(He) spectral class (Reindl et al. 2014b). Evolutionary tracks for H-deficient WDs (Althaus et al. 2009) labeled with the respective masses in M_⊙ are plotted for comparison. Four variants of the PG 1159–DO transition limit (labeled A–D, Unglaub & Bues 2000) are indicated.