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We re-analysed the carbon and oxygen isotopic ratios in the atmospheres ofthe two bright K giants Arcturus and Aldebaran. Previous determinations oftheir 16O/18O ratios showed a rough agreement with FDU expectations; however,the estimated 16O/17O and 12C/13C ratios were lower than in the canonicalpredictions. These anomalies are interpreted as signs of the occurrence ofnon-convective mixing episodes. We re-investigated this issue in order toverify whether the observed data can be reproduced in this hypothesis and ifthe well determined properties of the two stars can help us in fixing theuncertain parameters characterizing non-convective mixing and its physicalnature. We used high-resolution infrared spectra to derive the 12C/13C and16O/17O/18O ratios from CO molecular lines near 5 mu. We also reconsidered thedetermination of the stellar parameters to build the proper atmospheric andevolutionary models. We found that both the C and the O isotopic ratios for thetwo stars considered actually disagree with pure FDU predictions. Thisreinforces the idea that non-convective transport episodes occurred in them. Byreproducing the observed elemental and isotopic abundances with the help ofparametric models of nucleosynthesis and mass circulation, we derivedconstraints on the properties of non convective mixing. We find that very slowmixing is incapable of explaining the observed data, which require a fasttransport. Circulation mechanisms with speeds intermediate between thosetypical of diffusive and of convective mixing should be at play. We howeverconclude with a word of caution on the conclusions possible at this stage, asthe parameters for the mass transport are rather sensitive to the stellar massand initial composition.

Carbon and oxygen isotopic ratios in Arcturus and Aldebaran: Constraining the parameters for non convective mixing on the RGB