Location of the spectroscopically calibrated variable and nonvariable PG 1159 stars and variable [WCE] stars in the log T_eff − log g plane. PG 1159 stars with no variability data (N. V. D.) are depicted with hollow circles. Thin solid curves show the evolutionary tracks from Miller Bertolami & Althaus (2006) for stellar masses 0.530, 0.542, 0.565, 0.589, 0.609, 0.664, and 0.741 M_⊙. Parameterizations of the theoretical pulsational dipole (solid curves) and quadrupole (dashed curves) red and blue edges of the instability domain according to Córsico et al. (2006) are also displayed. The location of the GW Vir stars RX J2117 (red star), K 1−16 (red star), HS 2324 (blue plus), NGC 6905, NGC 1501, and NGC 2371 (magenta diamonds) are emphasized with large symbols.