Average carbon isotope ratios in clump stars of open clusters as a function of stellar turn-off mass. The result for Collinder 261 is indicated by a filled triangle and for Melotte 66 by a filled square. The previous result determined for Collinder 261 by Mikolaitis et al. (2012) is indicated by an open square. Open squares were also used to show other results from Mikolaitis et al. (2010, 2011a,b, 2012) and Tautvaišienė et al. (2000, 2005). Results from Smiljanic et al. (2009) are indicated by open triangles; from Luck (1994) – reversed open triangle; from Gilroy (1989) – open circles; from Katime Santrich et al. (2013) – open diamond. The solid lines (1DUP ST) represent the ¹²C /¹³C ratios predicted for stars at the first dredge-up with standard stellar evolutionary models of solar metallicity by Charbonnel & Lagarde (2010; blue upper line) and, more recently, Lagarde et al. (2012; black lower line). The blue dashed line (TH) shows the prediction when just thermohaline extra-mixing is introduced (Charbonnel & Lagarde 2010), and the black dashed line (TH+V) is for the model that includes both the thermohaline- and rotation-induced mixing (Lagarde et al. 2012), A typical error bar is indicated (Charbonnel & Lagarde 2010; Smiljanic et al. 2009; Gilroy 1989).