Surface gravity (log g) as a function of T_eff (Kiel diagram) for the stars in the Bok sample. The six spectral groups are indicated with symbols of different colors. Stars with IR excess are marked with an additional orange circle (see Sect. 4.1.2). The ZAEHB and TAEHB computed with STELUM for a core mass of 0.47 M_⊙ are shown with red dashed lines. The ZAEHB extension below 20 kK (solid red line) is from BaSTI models. Two BaSTI evolutionary tracks are shown in purple, the solid part represents the core-He burning phase, while the dotted part is the post-EHB phase. The dashed purple track is from a late-flasher model. The dashed-dotted red line is the ZAHeMS with the stellar masses indicated along the line. References for the theoretical models are listed in Sect. 4.1.1.