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 , as defined in 27 ) 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 \pm 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 \pm 0.01 .
The three RGB-a stars show , instead , a significantly lower \alpha -enhancement : < [ \alpha /Fe ] > = +0.10 \pm 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 .
( 32 ) .
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 .