Stellar evolution of globular cluster giants in which the external layers are CNO-enriched.

The evolution of typical globular cluster stars during the hydrogen shell burning phase is computed under hypothesis that the external layers of these stars have been polluted in the past by CNO-enriched material. When these stars evolve off the main sequence, convection spreads the CNO rich material into a large fraction of the star mass, reaches a maximum in mass extension and then recedes in front of the advancing hydrogen burning shell. When the shell encounters the chemical discontinuity left by the convective penetration, the star luminosity first recedes, then the evolution accelerates noticeably due to the sudden increase in the nuclear energy generation rate via the CNO cycle. The velocity of evolution becomes a factor 4 (or more) larger than before, in an interval of luminosity whose amplitude depends on the amount of the CNO-enrichment.