8 Conclusions

1.

The early AGB stars in NGC 1866 show lithium abundances which are roughly constant with increasing luminosity, consistent with the lithium dilution expected to have taken place during the previous evolutionary stages. The average abundance found is , implying, however, stronger than standard dilution. This result is confirmed by at least another early AGB star in NGC 2031, although the data derived from other stars analyzed in this cluster are inconclusive;

2.

We have detected three cool luminous AGB stars in NGC 1866. The faintest one (#3) shows a lithium abundance consistent with the remnant abundance expected from an AGB star at the beginning of this phase as a consequence of previous lithium dilution. The brightest one (#1) does not show any lithium and the upper limit derived ( ) suggests that this star plausibly is in the phase of HBB preceding lithium production (confirmed by the smaller mid-IR excess detected by ISOCAM). The other most luminous AGB star (#2) has a larger lithium abundance ( $\log N({\rm Li})\simeq 1.5\pm 0.5$), which we can attribute to production by HBB. On the basis of our models we would expect a few ($\sim$5) other AGBs in the field of NGC 1866, while none were found, neither luminous in the optical, nor in the mid-IR. This result put interesting constraints on the duration of the AGB phase and the severity of the mass loss during this phase.

3.

The most luminous star in NGC 2031 (#1) is found to be similar to star #1 in NGC 1866. The second brightest star in the K-band in this cluster (#2) has the largest lithium abundance in the sample, but Li production is not predicted at its derived luminosity (HBB not active). Further observations are needed to confirm the abundance analysis and the membership of this star.

We conclude that, though many points still remain unclear in the interpretation of the observations, we are in presence of an interesting sample of stars, whose further careful analysis can shed light on the expected evolutionary paths. Observations at a higher dispersion are needed for these stars to clarify their evolutionary status: for example, the presence of s-process elements in the spectra would be an important indicator of how many thermal pulses the stars have gone through. Our best guess is that they still are at the beginning of the TP phase, so that we should not expect a sensible s-process abundance enhancement.

Acknowledgements

CM, VT and FD acknowledge discussions with Paolo Ventura and Paolo Persi, and the support of the Italian Ministry of University, Scientific Research and Technology (MUIR) within the Cofin 1999 Project: "Stellar Dynamics and Stellar Evolution in Globular Clusters: a Challenge for the New Astronomical Instruments". PGL acknowledges support from grant PB97-1435-C02-02 from the Spanish Dirección General de Enseñanza Superior e Investigación Científica (DGESIC).