Theoretical predictions for the run of [C/N] _FDU as a function of age for selected metallicities. From top to bottom, at an age of 10 Gyr, [M/H] is equal to −2.27 (dotted line), −1.49 (short dashed), −1.27(long dashed), −0.66 (dot-dashed), −0.35 (thick solid), 0.06 (thin solid) and 0.26 dex (thin long dashed), respectively. Data for halo field stars and the labelled open clusters are also plotted (see text for details).