Expected characteristics of RR Lyrae stars as a function of the evolutive parameters are reported. Results from both evolutionary and pulsational investigations are collected in a suitable form, to show the general constraints to any interpretative analysis of the observations. It is shown that the spread in luminosity among the RR Lyrae stars results a function of the original chemical composition. On this basis a set of independent indications is found, suggesting that the globular cluster co Cen is more He-rich than M 3; agreement with the whole observational frame is attained if Yo, cen~0.35, Z~ c*, ~ 5 x 10-4 and YM a ~ 0.25, ZM 3 ~ 10- ~. No mass loss is needed to account for the RR Lyrae stars observed in co Cen. The results are discussed, and it is shown that M 13-type clusters can be just characterized by a larger value of Z in comparison with co Cert. It is suggested that variations in the original helium content of the order of A Y~0.1 and a correlation Z=Z(t) can account for some well-observed galactic globular clusters, without allowing for mass loss in the redder HB stars belonging to each cluster.