Derived ¹⁶O/¹⁷O vs. ¹²C/¹³C ratios in the carbon stars of different spectral types: N-type (blue), SC-type (green) and J-type (red). The position of the stellar masses on the figure approximately indivcates the value of the predicted ¹⁶O/¹⁷O ratio for each mass after the FDU for solar metallicity stellar models. The operation of extra mixing on the AGB phase would place a star in the region ¹²C/¹³C < 30 and ¹⁶O/¹⁷O > 1500 (see Fig. 7). The solar ratios are marked for a guide. A typical error bar is shown.