Combinations of isotopic C and O ratios for selected extra-mixing runs performed for α Tau and for a case without extra-mixing (blue spot) compared to observed data. The models and the meaning of the line types are the same as in Fig. 5. The small squares indicate the envelope isotopic mix reached by each mixing case when the model effective temperature is comparable with the observed one. At the same mixing depth, the higher the mixing rate Ṁ₆, the lower the value of the carbon isotopic ratio. Open stars are the isotopic ratios derived by Harris & Lambert (1984).