Zero Age Main Sequence (ZAMS) models with updated physical inputs are presented for selected assumptions about the chemical composition, covering the ranges 0.6 < M < 1.2 M., 0.0001 < Z < 0.04, 0.23 < Y < 0.34. The HR diagram location of the ZAMS as a function of Y and Z is discussed both in the theoretical (logL, logT(eff)) and in the observational (M(upsilon), B-V) diagrams, showing that the V magnitude presents an increased dependence on Z to be taken into account when discussing observational evidences. Analytical relations quantifying both these dependences are derived. Implications for the galactic helium to heavier elements enrichment Delta Y/Delta Z are finally discussed.
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