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Abundances of C and N are derived from features due to the CH G-band and tothe UV CN band measured on high resolution (R ~ 40,000) UVES spectra of morethan 40 dwarfs and subgiants in NGC 6397, NGC 6752 and 47 Tuc. Oxygenabundances (or upper limits) are available for all stars in the sample.Isotopic ratios 12C/13C were derived from the CH molecular band. This is thefirst determination of this ratio in unevolved dwarf stars in globularclusters. By enlarging the sample of subgiants in NGC 6397 studied in Grattonet al. (2001), we uncovered, for the first time, large variations in both Naand O also in this cluster. The origin of the chemical inhomogeneities must besearched for outside the stars under scrutiny. Our data indicate that inunevolved or slightly evolved stars in these clusters C abundances are low butnot zero, also in stars with large N-enhancements and O-depletions, and thatthe isotopic ratios 12C/13C are low, but never reach the equilibrium value ofthe CN-cycle. When coupled with the run of O and Na abundances, these findingspossibly require that, in addition to CNO burning and p-captures, sometriple-alpha process is also involved: previously evolved intermediate-mass AGBstars are then the most likely polluters.Comment: 16 pages, 18 figures, accepted for publication on A&

Abundances of C, N, O in slightly evolved stars in the globular clusters NGC 6397, NGC 6752 and 47 Tuc