Chemical abundances of blue straggler stars in Galactic Globular
  Clusters
  By using the high resolution spectrograph FLAMES@VLT we performed the first
systematic campaign devoted to measure chemical abundances of blue straggler
stars (BSSs). These stars, whose existence is not predicted by the canonical
stellar evolutionary theory, are likely the product of the interactions between
stars in the dense environment of Globular Clusters. Two main scenarios for BSS
formation (mass transfer in binary systems and stellar collisions) have been
proposed and hydrodynamical simulations predict different chemical patterns in
the two cases, in particular C and O depletion for mass transfer BSSs. In this
contribution, the main results for BSS samples in 6 Globular Clusters and their
interpretation in terms of BSS formation processes are discussed. For the first
time, evidence of radiative levitation in the shallow envelopes of BSSs hotter
than $\sim$8000 K has been found. C and O depletion for some BSSs has been
detected in 47 Tucanae, M30 and $\omega$ Centauri thus suggesting a mass
transfer origin for these BSSs. The low percentage of CO-depleted BSSs suggests
that depletion might be a transient phenomenon.
