  Marigo (2002) has highlighted the crucial importance of molecular opacities
in modelling the evolution of AGB stars at varying surface C/O ratio. In
particular, it has been shown the large inadequacy of solar-scaled opacities
when applied to models of carbon stars, and hence the need for correctly
coupling the molecular opacities to the current surface chemical composition of
AGB stars. The aim of the present follow-up study is to investigate the effects
of variable molecular opacities on the evolutionary properties of luminous AGB
stars with massive envelopes, i.e. with initial masses from ~3.5 Msun up to 5-8
Msun, which are predicted to experience both the third dredge-up and hot-bottom
burning. It is found that if the dredge-up of carbon is efficient enough to
lead to an early transition from C/O<1 to C/O>1, then hot-bottom burning may be
weakened, extinguished, or even prevented. The physical conditions for this
occurrence are analysed and a few theoretical and observational implications
are discussed. Importantly, it is found that the inclusion of variable
molecular opacities could significantly change the current predictions for the
chemical yields contributed by intermediate-mass AGB stars, with M~3.5 - 4.0
Msun that make as much as ~ 30-50 % of all stars expected to undergo hot-bottom
burning.
