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We present FLAMES/GIRAFFE spectroscopy obtained at the Very Large Telescope(VLT). Using these observations we have been able for the first time to observethe Li I doublet in the Main Sequence stars of a Globular Cluster. We alsoobserved Li in a sample of Sub-Giant stars of the same B-V colour. Our finalsample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SGand MS span the same temperature range we find that the equivalent widths ofthe Li I doublet in SG stars are systematically larger than those in MS stars,suggesting a higher Li content among SG stars. This is confirmed by ourquantitative analysis which makes use of both 1D and 3D model atmospheres. Wefind that SG stars show, on average, a Li abundance higher by 0.1 dex than MSstars. We also detect a positive slope of Li abundance with effectivetemperature, the higher the temperature the higher the Li abundance, both forSG and MS stars, although the slope is slightly steeper for MS stars. Theseresults provide an unambigous evidence that the Li abundance changes withevolutionary status. The physical mechanisms that contribute to this are notyet clear, since none of the proposed models seem to describe accurately theobservations. Whether such mechanism can explain the cosmological lithiumproblem, is still an open question.

Lithium abundances of Main Sequence and Sub-Giant stars in the Globular Cluster NGC 6397