The 12C/13C-ratio is an important tracer of stellar evolution and nucleosynthesis. Current theoretical models predict that Asymptotic Giant Branch (AGB) stars will evolve from the normal spectral type M, via the intermediate S-type, into carbon stars (C-type), as more and more carbon is brought up to the surface through the reoccurring dredge-up events.
Since the 12C-isotope is the main carbon isotope synthesized, a coeval evolution of the 12C/13C-ratio is expected. In addition, although the observational constraints are sparse, the 12C/13C-ratios of AGB stars and the planetary nebulae (PNe) seem to differ, perhaps limiting the properties required to reach this final evolutionary stage. Accurate estimates of the 12C/13C-ratio along the AGB will increase our understanding of the processes that lead to the formation of carbon stars, as well as the formation of PNe.
We have estimated 12CO/13CO-ratios in a large sample of AGB stars of different chemical type and I will present the results together with the recent ALMA observations of the carbon star R Scl.