Distribution of the oxygen-rich post-AGB stars studied here on the H-R diagram, according to the data reported in Table 1. The evolutionary tracks used for the interpretation of the individual objects are also shown, including the late AGB and the post-AGB phase; the tracks considered here have a metallicity of Z = 0.004 (solid lines) and Z = Z_⊙ (dashed line), and their mass at the beginning of the AGB phase is reported on the right side.