Mean carbon-to-nitrogen ratios in stars of open clusters as a function of stellar turn-off mass. The diamond represents Trumpler 20 stars, the triangle is for NGC 4815, and the square is for NGC 6705 stars. The solid lines represent the C/N ratios predicted for stars at the first dredge-up with standard stellar evolutionary models by Charbonnel & Lagarde (2010; blue upper line) and, more recently, Lagarde et al. (2012; black lower 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), see Sect. 3.1 for more explanations.