  We use the available data for Magellanic Cloud carbon stars to constrain the
efficiency of the third dredge-up process in TP-AGB models. We show that star
counts in LMC clusters provide quite stringent limits to the lifetime of the
C-star phase, with a duration between 2 and 3 Myr for stars in the mass range
from 1.5 to 2.8 Msun. Together with the luminosity functions of field C stars,
this information allows us to re-calibrate the third dredge-up parameters log
T_b_dred and lambda in TP-AGB models that include variable molecular opacities
(Marigo 2002). Preliminary results are presented here.
