HRD of the Galactic post-AGB and post-RGB binary sample. Different markers denote whether the orbital periods of the objects are known. The T_eff error bars of ±250 K were omitted for the sake of clarity. Evolutionary tracks of our single-star models with Z = 0.02, M_i = 1.5 M_⊙ and Z = 0.00142, M_i = 1.0 M_⊙ are shown in green and black, respectively, up to the final AGB evolution. In addition, we show a few illustrative post-RGB evolution tracks with initial orbital periods ranging from 3.95 days to 258 days (from bottom to top). These models have donors identical to their starting point on the corresponding single-star evolutionary track and have mass ratios equal to unity prior to mass transfer. Orbital periods of the sample binaries as well as the orbital period evolution of the models are shown by the colour bar. The dashed grey line corresponds to the RGB-tip luminosity of 2500 L_⊙.