  We present Li, Na, Al and Fe abundances of 199 lower red giant branch stars
members of the stellar system Omega Centauri, using high-resolution spectra
acquired with FLAMES at the Very Large Telescope. The A(Li) distribution is
peaked at A(Li) ~ 1 dex with a prominent tail toward lower values. The peak of
the distribution well agrees with the lithium abundances measured in lower red
giant branch stars in globular clusters and Galactic field stars. Stars with
A(Li) ~ 1 dex are found at metallicities lower than [Fe/H] ~ -1.3 dex but they
disappear at higher metallicities. On the other hand, Li-poor stars are found
at all the metallicities. The most metal-poor stars exhibit a clear Li-Na
anticorrelation, with about 30% of the sample with A(Li) lower than ~ 0.8 dex,
while in normal globular clusters these stars represent a small fraction. Most
of the stars with [Fe/H] > -1.6 dex are Li-poor and Na-rich. The Li depletion
measured in these stars is not observed in globular clusters with similar
metallicities and we demonstrate that it is not caused by the proposed helium
enhancements and/or young ages. Hence, these stars formed from a gas already
depleted in lithium. Finally, we note that Omega Centauri includes all the
populations (Li-normal/Na-normal, Li-normal/Na-rich and Li-poor/Na-rich stars)
observed, to a lesser extent, in mono-metallic GCs.
