Upper panel: run of the [C/N] abundance ratio within a 0.9 M_⊙ stellar model ([M/H] = −0.35, scaled solar metal mixture), before the occurrence of the FDU (thick line) and right after the completion of the FDU (thin line). The horizontal axis displays the local fractional mass (mass enclosed within a distance r from the centre divided by the total mass). Lower panel: as upper panel but for the individual mass fractions of N (dashed lines) and C (solid lines).