ADVANCED EVOLUTIONARY PHASES OF LOW-MASS STARS - THE ROLE OF THE ORIGINAL HELIUM

We present evolutionary He burning models which cover both the HE and AGE evolutionary phases, as computed for the metallicities Z = 10(-4), 10(-3) and for two assumptions on the value of the original He conten t Y = 0.20, 0.27. We found that both the difference in luminosity betw een the AGE and the HE, and the number ratio of AGE to HE stars appear quite unaffected by the amount of original He, allowing a relevant te st for the efficiency of internal mixing during the central He burning phase. Data concerning HE evolution have been connected with the evol utionary times of the corresponding RGB stars to recalibrate the param eter R as a function of Y for the two investigated metallicities. Comp arisons with previous calibrations disclose variations which are not n egligible, and which should be taken into account when R is used to de rive constraints on the amount of cosmological helium. The critical ro le of the HE luminosity on the evaluation of R is briefly discussed.