Carbon isotopes in globular clusters down to the bump in the luminosity function

We find that the C-12/C-13 ratio evolves from high values (>20) below the bump in the luminosity function (BLF) to near the equilibrium value of the CNO cycle above the BLF in the globular clusters (GCs) NGC 6528 and M4. This is the first time that the predicted decline of the C-12/C-13 ratios due to the extra mixing at the BLF is detected in a GC. In M4, a slight decline from C-12/C-13 = 10 just above the BLF at M-nu = +0.5 to C-12/C-13 = 4 at M-nu = -0.6 is detected, suggesting that some additional mixing may occur beyond the BLF in this cluster. Isotope ratios are measured and found to be constant in the GCs NGC 6553 and 47 Tucanae down to just above the BLF of those GCs.