The effective temperatures and surface gravities derived for our target stars (formal errors multiplied by 2, see text for details). Helium-poor and helium-rich stars are marked by open squares and filled triangles, respectively. The stars with super-solar helium are shown with the parameters derived from models without C/N enhancement. The solid lines mark the canonical HB locus (Y = 0.23) and the dashed lines mark the helium-enriched HB locus (Y = 0.38, see text for details). The tracks for a late hot flasher (same line types as for the ZAHB) show the evolution of these stars from the zero-age HB (ZAHB) towards helium exhaustion in the core (terminal-age HB = TAHB). The dotted line connects the series of ZAHB models computed by adding a hydrogen-rich layer to the surface of the canonical ZAHB model of the late hot flasher. The open circles mark – with decreasing temperature – hydrogen layer masses of 0,10^-7,10^-6,10^-5, and 10^-4 M_⊙ (for details see Moehler et al. 2002).