  We have obtained high-resolution, high S/N spectra for six red giants in
omega Cen: three belonging to the recently discovered, metal-rich Red Giant
Branch (RGB-a, Pancino et al. 2000) and three to the metal intermediate
population (RGB-MInt). Accurate Iron, Copper and alpha-elements (Ca and Si)
abundances have been derived and discussed. In particular, we have obtained the
first direct abundance determination based on high-resolution spectroscopy for
the RGB-a population, <[Fe/H]>=-0.60+/-0.15. Although this value is lower than
previous estimates based on Calcium triplet measurements, we confirm that this
population is the most metal rich in omega Cen. In addition, we have found a
significant difference in the alpha-elements enhancement of the two
populations. The three RGB-MInt stars have the expected overabundance, typical
of halo and globular clusters stars: <[alpha/Fe]>=+0.29+/-0.01. The three RGB-a
stars show, instead, a significantly lower alpha-enhancement:
<[alpha/Fe]>=+0.10+/-0.04. We have also detected an increasing trend of [Cu/Fe]
with metallicity, similar to the one observed for field stars by Sneden et al.
(1991). The observational facts presented in this letter, if confirmed by
larger samples of giants, are the first indication that supernovae type Ia
ejecta have contaminated the medium from which the metal rich RGB-a stars have
formed. The implications for current scenarios on the formation and evolution
of omega Cen are briefly discussed.
