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We estimate the empirical R parameter in 26 Galactic Globular Clusterscovering a wide metallicity range, imaged by WFPC2 on board the HST. Theimproved spatial resolution permits a large fraction of the evolved stars to bemeasured and permits accurate assessment of radial populaton gradients andcompleteness corrections. In order to evaluate both the He abundance and the Heto metal enrichment ratio, we construct a large set of evolutionary models byadopting similar metallicities and different He contents. We find an absoluteHe abundance which is lower than that estimated from spectroscopic measurementsin HII regions and from primordial nucleosynthesis models. This discrepancycould be removed by adopting a C12O16 nuclear cross section about a factor oftwo smaller than the canonical value, although also different assumptions formixing processes can introduce systematical effects. The trend in the Rparameter toward solar metallicity is consistent with an upper limit to the Heto metal enrichment ratio of the order of 2.5.Comment: accepted for pubblication on Ap

On the Helium Content of Galactic Globular Clusters via theR‐Parameter