  We present deep and precise photometry (F435, F625W, F658N) of Omega Cen
collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space
Telescope (HST). We have identified ~ 6,500 white dwarf (WD) candidates, and
the ratio of WD and Main Sequence (MS) star counts is found to be at least a
factor of two larger than the ratio of CO-core WD cooling and MS lifetimes.
This discrepancy is not explained by the possible occurrence of a He-enhanced
stellar population, since the MS lifetime changes by only 15% when changing
from a canonical (Y=0.25) to a He-enhanced composition (Y=0.42). The presence
of some He-core WDs seems able to explain the observed star counts. The
fraction of He WDs required ranges from 10% to 80% depending on their mean mass
and it is at least five times larger than for field WDs. The comparison in the
Color Magnitude Diagram between theory and observations also supports the
presence of He WDs. Empirical evidence indicates that He WDs have been detected
in stellar systems hosting a large sample of extreme horizontal branch stars,
thus suggesting that a fraction of red giants might avoid the He-core flash.
