Evolutionary parameters in observed horizontal and asymptotic branches

Stellar evolution during the Horizontal and Asymptotic Branch phases has been computed for selected models with realistic evolutionary parameters, under the assumption Z = 10−3. The models have been used to determine the helium dependence of some observable evolutionary characteristics of globular clusters. Comparison of the theoretical evolutionary tracks with observational data supports the full efficiency of semi-convection during the HB phase. Using the fully semi-convective models, original helium abundances are estimated for a number of globular clusters. The expected distribution of fundamental periods of RR Lyrae variables in globular clusters is investigated allowing for a wide variation in the amount of differential mass loss. A Y-dependent relation between the morphology of the HB and the mean fundamental period of RR Lyrae pulsators is found.