  We analyze spectra of 18 stars belonging to the faintest subgiant branch in
omega Centauri (the SGB-a), obtained with GIRAFFE@VLT at a resolution o R~17000
and a S/N ratio between 25 and 50. We measure abundances of Al, Ba, Ca, Fe, Ni,
Si, and Ti and we find that these stars have <[Fe/H]>=-0.73 +/- 0.14 dex,
similarly to the corresponding red giant branch population (the RGB-a). We also
measure <[alpha/Fe]>=+0.40 +/- 0.16 dex, and <[Ba/Fe]>=+0.87 +/- 0.23 dex, in
general agreement with past studies. It is very interesting to note that we
found a uniform Al abundance, <[Al/Fe]>=+0.32 +/- 0.14 dex, for all the 18
SGB-a stars analysed here, thus supporting past evidence that the usual
(anti-)correlations are not present in this population, and suggesting a non
globular cluster-like origin of this particular population. In the dwarf galaxy
hypothesis for the formation of omega Cen, this population might be the best
candidate for the field population of its putative parent galaxy, although some
of its properties appear contradictory. It has also been suggested that the
most metal-rich population in omega Cen is significantly enriched in helium. If
this is true, the traditional abundance analysis techniques, based on model
atmospheres with normal helium content, might lead to errors. We have computed
helium enhanced atmospheres for three stars in our sample and verified that the
abundance errors due to the use of non-enhanced atmospheres are negligible.
Additional, indirect support to the enhanced helium content of the SGB-a
population comes from our Li upper limits.
