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Improved isotopic ratio determinations in IRC+10216, the progenitor mass and the s process

We present an improved determination of the chlo- rine isotopic ratio in the circumstellar envelope IRC+10216. This determination is the first clear evidence that in this cir- cumstellar envelope elements heavier than the CNO do show deviations from the solar isotopic ratios, thus allowing a de- tailed comparison with quantitative model predictions. We have compared these data, together with other measurements for Mg and CNO isotopes, with predictions of AGB stellar models of solar metallicity and initial masses 1.5, 3 and 5 M computed with the FRANEC evolutionary code. The comparison shows excellent agreement for the results of a low mass AGB model and remarkable disagreement for those of the 5 M star. We briefly discuss the impact that model uncertainties can have on our conclusions.