Surface evolution of the ¹²C/¹³C ratio in a 2 M_⊙ model of solar metallicity. The different colours represent the different evolutionary stages (HB for core-hydrogen burning, HeB for core-helium burning). The solid line corresponds to the standard model computed without overshoot (f_top = 0) and thus without PIE. The dashed line shows the same model but computed with f_top = 0.12 and hence experiencing a PIE during the early AGB phase (denoted by the green circle). The two dotted horizontal lines delineate the range of ¹²C/¹³C ratios observed in J-type stars (Table 1).