Context :

Aims : One of the puzzling findings in the study of the chemical evolution of ( post- ) AGB stars is why very similar stars ( in terms of metallicity , spectral type , infrared properties , etc… ) show a very different photospheric composition .
We aim at extending the still limited sample of s-process enriched post-AGB stars , in order to obtain a statistically large enough sample that allows us to formulate conclusions concerning the 3rd dredge-up occurrence .

Methods : We selected two post-AGB stars on the basis of IR colours indicative of a past history of heavy mass loss : IRAS 08281-4850 and IRAS 14325-6428 .
They are cool sources in the locus of the Planetary Nebulae ( PNe ) in the IRAS colour-colour diagram .
Abundances of both objects were derived for the first time on the basis of high-quality UVES and EMMI spectra , using a critically compiled line list with accurate log ( gf ) values , together with the latest Kurucz model atmospheres .

Results : Both objects have very similar spectroscopically defined effective temperatures of 7750 - 8000 K. They are strongly carbon and s-process enriched , with a C/O ratio of 1.9 and 1.6 , and an [ ls/Fe ] of + 1.7 and + 1.2 , for IRAS 08281-4850 and IRAS 14325-6428 respectively .
Moreover , the spectral energy distributions ( SEDs ) point to heavy mass-loss during the preceding AGB phase .

Conclusions : IRAS 08281-4850 and IRAS 14325-6428 are prototypical post-AGB objects in the sense that they show strong post 3rd dredge-up chemical enrichments .
The neutron irradiation has been extremely efficient , despite the only mild sub-solar metallicity .
This is not conform with the recent chemical models .
The existence of very similar post-AGB stars without any enrichment emphasizes our poor knowledge of the details of the AGB nucleosynthesis and dredge-up phenomena .
We call for a very systematic chemical study of all cool sources in the PN region of the IRAS colour-colour diagram .