(abridged) Recent spectroscopic and photometric observations show theexistence of various generations of stars in GCs, differing in the abundancesof products of H-burning at high temperatures (the main final product beingHe). It is important to study the connections between stars properties and Hecontent. We consider here the about 1400 stars on the Red Giant Branch (RGB)observed with FLAMES@VLT in 19 Galactic GCs, part of out Na-O anticorrelationprojet. Stars with different He are expected to have different temperatures(i.e. colours), slightly different [Fe/H], and different luminosity levels ofthe RGB bump. All these differences are small, but our study has the necessaryprecision, good statistics, and homogeneity to detect them. We also computedsuitable sets of stellar models (BaSTI) for various assumptions about theinitial helium content. Differences in observable quantities that can beattributed to variations in He content are generally detectable between starsof the Primordial (P, first-generation) and Extreme (E, second-generation)populations, but not between the Primordial and Intermediate ones (I). The onlyexception (differences are significant also between P and I populations) isNGC2808, where three populations are clearly separated also on the MainSequence and the Horizontal Branch. The average enhancement in the He massfraction Y between P and E stars is about 0.05-0.11, depending on theassumptions. The differences in Y, for NGC2808 alone, are about 0.11-0.14between P and I stars, and about 0.15-0.19 between P and E stars, againdepending on the assumptions. The RGB bump luminosity of first andsecond-generation stars has different levels; the implied Y difference is moredifficult to quantify, but is in agreement with the other determinations.
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