Mean C/N ratio of clump stars in open clusters as a function of turn-off mass. Three open clusters from T15 are labeled as open squares, while NGC 6802 is the filled square. The solid lines represent the C/N ratios predicted by the first dredge-up with standard stellar evolutionary models in Charbonnel & Lagarde (2010; blue solid line) or Lagarde et al. (2012; black solid line). The blue dashed line shows the prediction when just thermohaline extra-mixing is introduced (Charbonnel & Lagarde 2010), and the black dashed line is for the model that includes both the thermohaline and rotation induced mixing (Lagarde et al. 2012).