Involved people at OAB: Bellazzini, Ferraro.
In the framework of a global study of the formation and evolution of the
galactic halo, a detailed photometric and spectroscopic study of the stellar
population in 
Cen has been recently started
[in collaboration with E. Pancino (Univ. BO), G. Piotto (Univ. PD),
L. Pasquini (ESO)].
Cen is the most
luminous and massive GGCs, it is one of the very rare cases of self-enriching
GC and it is significantly flattened by internal rotation. Indeed a very
peculiar and interesting case of study.
The first data set acquired at the ESO 2.2m with the WFI did show
a spectacular and unexpected result: in the 
CMD we found for the very
first time an anomalous, narrow Red Giant Branch (RGB) significantly redder
than the bulk of the normal RGB stars [Pancino et al. (2000, ApJ, 534, L83)],
indicating not just a spread in metallicity, that was already known, but the
presence of a separate stellar population at higher metallicity. The
interpretation is not unambiguous, but may support the accretion event of
metal-rich material at a later stage during the formation of the cluster, that
may or may not have had the time for dynamical relaxation.
Spectroscopic observations of a set of these stars
are already secured and will be soon published.