Average carbon isotope ratios in clump stars of open clusters as a function of stellar TO mass. Red square indicates the value of IC 4756. Red open squares indicate results of previously investigated open clusters by Tautvaišienė et al. (2000, 2005, 2016); Mikolaitis et al. (2010, 2011a,b, 2012); Drazdauskas et al. (2016a,b). Other symbols include results from Gilroy (1989) – pluses, Luck (1994) – open circles, Smiljanic et al. (2009) – green crosses, 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; black solid line) and Lagarde et al. (2012; blue solid line). The short-dashed line (TH) shows the prediction when only thermohaline extra-mixing is introduced (Charbonnel & Lagarde 2010), and the long-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 (Gilroy 1989; Smiljanic et al. 2009; Charbonnel & Lagarde 2010).