A new calibration of the RGB Tip as a Standard Candle

We have obtained an accurate estimate of the absolute I magnitude of the Tip of the Red Giant Branch (M-I(T RGB)) for the globular cluster omega Centauri, based (a), on the largest photometric database ever assembled for a globular, by Pancino et al. (2000), and (b) on a direct distance estimate for omega Centauri, recently obtained by Thompson et al. (2001) from a detached eclipsing binary. The derived value MTITRGB = -4.04+/-0.12 provides, at present, the most accurate empirical zero-point for the calibration of the M-I(T RGB)- [Fe/H] relation, at [Fe/H]similar to -1.7.