  Carbon stars are known to exhibit systematically redder near-infrared colours
with respect to M-type stars. In the near-infrared colour-magnitude diagrams
provided by the 2MASS and DENIS surveys, the LMC C-type stars draw a striking
red tail, well separated from the sequences of O-rich giants. So far, this
conspicuous feature has been absent from any set of available isochrones, even
the few existing ones that include the TP-AGB evolution of low- and
intermediate-mass stars. To investigate such issue we simulate the complete
2MASS Ks vs.(J-Ks) data towards the LMC by means of a population synthesis
approach, that relies on extended libraries of published stellar evolutionary
tracks, including the TP-AGB phase. The simulations provide quite a detailed
description of the several vertical fingers and inclined sequences seen in
2MASS data, due to both Galactic foreground and LMC O-rich stars. Instead, as
mentioned, the red tail of C-stars sets a major difficulty: we find that TP-AGB
models with solar-scaled molecular opacities, the usual assumption of existing
AGB calculations, do not succeed in reproducing this feature. Our tests
indicate that the main reason for this failure should not be ascribed to
empirical Teff - (J-K) transformations for C-type stars. Instead, the
discrepancy is simply removed by adopting new evolutionary models that account
for the changes in molecular opacities as AGB stars get enriched in carbon via
the third dredge-up (Marigo 2002). In fact, simulations that adopt these models
are able to reproduce, for the first time, the red tail of C-stars in
near-infrared CMDs. Finally, we point out that these simulations also provide
useful indications about the efficiency of the third dredge-up process, and the
pulsation modes of long-period variables.
