  Using deep high-resolution multi-band images taken with the Very Large
Telescope and the Hubble Space Telescope, we discovered a new anomalous
sequence in the Color Magnitude Diagram of omega Cen. This feature appears as a
narrow, well-defined Sub Giant Branch (SGB-a), which merges into the Main
Sequence of the dominant cluster population at a magnitude significantly
fainter than the cluster Turn-Off (TO). The simplest hypothesis assumes that
the new feature is the extension of the anomalous Red Giant Branch (RGB-a)
metal-rich population discovered by Lee et al. (1999) and Pancino et al.
(2000). However, under this assumption the interpretation of the SGB-a does not
easily fit into the context of a self-enrichment scenario within omega Cen. In
fact, its TO magnitude, shape and extension are not compatible with a young,
metal-rich population, as required by the self-enrichment process. The TO level
of the SGB-a suggests indeed an age as old as the main cluster population,
further supporting the extra-cluster origin of the most metal rich stars, as
suggested by Ferraro, Bellazzini & Pancino (2002). Only accurate measurements
of radial velocities and metal abundances for a representative sample of stars
will firmly establish whether or not the SGB-a is actually related to the RGB-a
and will finally shed light on the origin of the metal rich population of omega
Cen.
