these empirical results, we verified by performing a number of tests (see Appendix A) how adequate is the use of ’standard’ $\alpha$-enhanced spectra to determine bolometric corrections in this temperature regime. We found that results obtained by using standard $\alpha$-enhanced bolometric corrections, are unaffected by the presence of the observed CNONa anti-correlations.

Regarding the effect of radiative levitation, spectroscopic investigations by Behr (2003), Moehler et al. (2003), Fabbian et al. (2005) have disclosed that - similar to the case of NGC2808 - in these three clusters HB stars hotter than $\sim$12000 K display surface abundances of metal species like Fe, Cr and Ti around solar or higher, due to radiative levitation in the atmosphere. Also, the surface He abundances are depleted by a factor of ten or more as a consequence of gravitational settling. Following the approach used in Paper I, we mimic this effect using ’standard’ $\alpha$-enhanced HB stellar models by applying bolometric corrections appropriate for \[Fe/H\]=0.0 (and scaled-solar mixture) when ${\rm T_{\rm eff}}$ is above 12000 K. This is of course a crude approximation, which we are forced to adopt because of the lack of extended grids of both HB stellar evolution and atmosphere models with a large 