Observing Third Dredge Up in NGC 1846

In this contribution we present the first measurements of the C/O ratio for a sample of AGB stars in the LMC globular cluster NGC 1846. From a variability study we set strong constraints on the mass of the cluster stars, which is an important quantity for comparison with evolutionary models. Variations of the surface abundance both with luminosity and pulsation properties were found and the various relevant factors are discussed. We find a good agreement with our synthetic models if we assume [O/Fe] = ±0.2 dex. The F abundance seems to increase with the C/O ratio. As variability plays a key role on the AGB we give some preliminary results from our study of abundance determination in the dynamical case at the end of this paper.