Evolution up to the RGB tip of C and O isotopic ratios in the 3 models with extra mixing described in the text. The maximum temperature attained by extra mixing is T_max = 20, 21, and 23 MK (black, red, and blue lines, respectively). In all cases, the fluid velocity in the extra-mixing zone is about 1/100 of that in the fully convective envelope. The evolutionary timescale has been properly rescaled to t_f = 2.91 Gyr, which corresponds to the onset of the core-He burning. The FDU starts at log t/ (t−t_f) ~ 1, while the extra-mixing process is activated just after log t/ (t−t_f) ~ 1.7, namely, the epoch of the RGB bump.