Position of our programme stars in the T_eff–logg diagram, in comparison with other PG1159 stars (black squares) and DO white dwarfs (red circles). Blue lines are VLTP post-AGB evolutionary tracks (labelled with stellar mass in M_⊙) from Miller Bertolami & Althaus (2006). Indicated is the PG1159 wind limit according to Unglaub & Bues (2000). No PG 1159 stars are found below it because radiation-driven stellar winds become so weak that they can no longer prevent gravitational settling of heavy elements.