Average ¹²C/¹³C ratios in clump stars of open clusters as a function of stellar turn-off mass. Our work is represented by coloured triangles and bars indicating a star-to-star scatter. Open squares are for determinations by Drazdauskas et al. (2016), Mikolaitis et al. (2010, 2011a,b, 2012), and Tautvaišienė et al. (2000, 2005), crosses for Smiljanic et al. (2009), a diamond for Santrich et al. (2013), a circle is for Luck (1994), and plus signs for Gilroy (1989). The solid lines represent standard dredge-up models (Eggleton et al. 2008; Charbonnel & Lagarde 2010; Lagarde et al. 2012), the dash-dotted line represents the thermohaline mixing model by Eggleton et al. (2008), the short-dashed line is for a model including thermohaline mixing Charbonnel & Lagarde (2010), and the long-dashed line represents a model that includes both the thermohaline and rotational effects (Lagarde et al. 2012). A typical error bar is indicated (Smiljanic et al. 2009; Gilroy 1989).