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We report for the first time Na and O abundances from high-resolution, highS/N echelle spectra of 20 red giants in NGC 2808, taken as part of the ScienceVerification program of the FLAMES multi-object spectrograph at the ESO VLT. Inthese stars, spanning about 3 mag from the red giant branch (RGB) tip, largevariations are detected in the abundances of oxygen and sodium, anticorrelatedwith each other; this is a well known evidence of proton-capture reactions athigh temperatures in the ON and NeNa cycles. One star appears super O-poor; ifthe extension of the Na-O anticorrelation is confirmed, NGC 2808 might reach Odepletion levels as large as those of M 13. This result confirms our previousfindings based on lower resolution spectra (Carretta et al. 2003) of a largestar-to-star scatter in proton capture elements at all positions along the RGBin NGC 2808, with no significant evolutionary contribution. Finally, theaverage metallicity for NGC 2808 is [Fe/H]= -1.14 +/- 0.01 dex (rms=0.06) from19 stars.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter

Star-to-Star Na and O Abundance Variations along the Red Giant Branch in NGC 2808