Positions of the hot post-AGB stars in the log T_eff − log g diagram. The blue lines denote the post-HB evolutionary tracks according to Dorman et al. (1993) with core mass of 0.49, 0.5, 0.51 and 0.53 M_⊙ and the red line denote the zero-age horizontal branch (ZAHB). The black lines denote the post-AGB evolutionary tracks according to Blöcker (1995) for core stellar masses of 0.605, 0625, 0.696, 0.836, and 0.940 M_⊙ and the green lines represent the post-AGB evolutionary tracks according to Schönberner (1983) for core stellar masses of 0.546 and 0.565 M_⊙. For comparison, we include some sdOB stars (blue stars and filled blue circles) of the samples of Heber (1986) and Ahmad & Jeffery (2003).