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The elemental abundances of ten planetary nebulae, derived with high accuracyincluding ISO and IUE spectra, are analysed with the aid of syntheticevolutionary models for the TP-AGB phase. Model prescriptions are varied untilwe achieve the simultaneous reproduction of all elemental features, whichallows placing important constraints on the characteristic masses andnucleosynthetic processes experienced by the stellar progenitors. First of all,it is possible to separate the sample into two groups of PNe, one indicatingthe occurrence of only the third dredge-up during the TP-AGB phase, and theother showing also the chemical signature of hot-bottom burning. The formergroup is reproduced by stellar models with variable molecular opacities (seeMarigo 2002), adopting initial solar metallicity, and typical efficiency of thethird dredge-up 0.3-0.4. The latter group of PNe, with extremely high Hecontent 0.15<=He/H<=0.20 and marked oxygen deficiency, is consistent withoriginal sub-solar metallicity (i.e. LMC composition). Moreover, we are able toexplain quantitatively both the N/H-He/H correlation and the N/H-C/Hanti-correlation, thus solving the discrepancy pointed out long ago by Becker &Iben (1980). This is obtained only under the hypothesis that intermediate-massTP-AGB progenitors (M > 4.5-5.0 Mo) with LMC composition have suffered a numberof very efficient, carbon-poor, dredge-up events. Finally, the neon abundancesof the He-rich PNe can be recovered by invoking a significant production of22Ne during thermal pulses, which would imply a reduced role of the22Ne(alpha,n)25Mg reaction as neutron source to the s-process nucleosynthesisin these stars.Comment: 22 pages, accepted for publication in Astronomy & Astrophysic

Probing AGB nucleosynthesis via accurate Planetary Nebula abundances