Hertzsprung-Russell diagram of post-AGB stellar models used in nebular simulations. They are from Schönberner (1983) and Schönberner & Blöcker (1993) and interpolated (0.595 and 0.585 M_⊙). The progress of stellar evolution along each track is indicated by time marks at 0.3 × 10³, 1 × 10³, 3 × 10³, 10 × 10³, and 30 × 10³ years. The “transition time” from the tip of the AGB to T_eff = 20 000 K is also given for each model sequence and is fully considered for the positioning of the time marks. The colour coding of the individual post-AGB tracks is kept the same throughout the whole paper.