Abundance variations on the lower giant branch of omega Centauri

Photoelectric photometry on the UBV, RI, and DDO systems is presented for stars on the lower giant branch of ω Centauri. Analysis of these data leads to the following conclusions: Differential reddening is small across the cluster (ΔE (B-V) approximately-less-than0.04). There exists a range of surface composition in the giants at the level of the horizontal branch which may be due either to abundance inhomogeneities within the cluster (which presumably existed at the end of the epoch of star formation) or to the mixing of processed materials from the interiors to the surfaces of these objects. Low-dispersion spectra (50 A mm-1) of some of these objects show relatively strong CN features indicating that the mixing hypothesis is probably the correct explanation