  Having shown in a recent paper that the main sequence of omega Centauri is
split into two distinct branches, we now present spectroscopic results showing
that the bluer sequence is_less_ metal-poor. We have carefully combined
GIRAFFE@VLT spectra of 17 stars on each side of the split into a single
spectrum for each branch, with adequate S/N to show clearly that the stars of
the blue main sequence are less metal poor by 0.3 dex than those of the
dominant red one. From an analysis of the individual spectra, we could not
detect any abundance spread among the blue main sequence stars, while the red
main sequence stars show a 0.2 dex spread in metallicity. We use
stellar-structure models to show that only greatly enhanced helium can explain
the color difference between the two main sequences, and we discuss ways in
which this enhancement could have arisen.
