Theoretical zero age main sequences revisited

Zero Age Main Sequence (ZAMS) models with updated physical inputs are prese nted 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 the oretical (logL, logT(eff)) and in the observational (M-upsilon, B-V) diagra ms, showing that the V magnitude presents an increased dependence on Z to b e taken into account when discussing observational evidences. Analytical re lations quantifying both these dependences are derived. Implications for th e galactic helium to heavier elements enrichment Delta Y/Delta Z are finall y discussed.