T_eff − log g diagram. The grey circles mark sdBs from the SPY project (Lisker et al. 2005). The helium main sequence (HeMS) is plotted and evolutionary tracks have been calculated by Han et al. (2002) for EHB masses of 0.7 M_⊙ (dotted lines), 0.6 M_⊙ (short-dashed lines), and 0.5 M_⊙ (long-dashed lines). The tracks to the lower left mark models without a hydrogen envelope, the tracks in the middle models with 0.001 M_⊙ envelope mass, and the tracks to the upper right models with 0.005 M_⊙ envelope mass.