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

Abundances of C and N are derived from features due to the CH G-band and to the UV CN band measured on high resolution ( R greater than or similar to 40 000) UVES spectra of more than 40 dwarfs and subgiants in NGC 6397, NGC 6752 and 47 Tuc. Oxygen abundances ( or upper limits) are available for all stars in the sample. Isotopic ratios C-12/C-13 were derived from the CH molecular band. This is the first determination of this ratio in unevolved dwarf stars in globular clusters. By enlarging the sample of subgiants in NGC 6397 studied in Gratton et al. ( 2001), we uncovered, for the first time, large variations in both Na and O in this cluster, too. The origin of the chemical inhomogeneities must be searched for outside the stars under scrutiny. Our data indicate that C abundances are low but not zero in unevolved or slightly evolved stars in these clusters, including in stars with large N-enhancements and O-depletions. The isotopic ratios C-12/C-13 are low, but never reach the equilibrium value of the CN-cycle. When coupled to the run of O and Na abundances, these findings may require that, in addition to CNO burning and p-captures, some triple-alpha process is also involved: previously evolved intermediate-mass AGB stars are then the most likely polluters.