Asymptotic giant branch (AGB) stars are among the most important gas and dust polluters of the Universe. The latest AGB evolutionary models take into account dust production in the circumstellar envelope of these stars, starting from a detailed computation of the main physical processes and chemical surface variations occurring in this evolutionary phase. Following the formation and growth of dust particles, they provide the unique possibility of interpreting the AGB population observed in resolved galaxies. The first application was for the Spitzer observations of dusty AGBs in the Magellanic Clouds, characterising carbon-rich and oxygen-rich stars in terms of initial mass, epoch of star formation, evolutionary time on the AGB and dust contribution. The same set of models are able to interpret the CNO surface abundances observed for the PNe of the same galaxies.

We report new calculations of the effective recombination coefficients for the nebular N ii and O ii lines and very deep spectroscopy of the bright planetary nebula (PN) NGC 7009.

Photoelectric UBV photometry of stars was carried out in the angular vicinity (radius = 15′) of 3 planetary nebulae. An extinction distance relation was constructed for the line of sight towards each planetary nebula. This relation was verified by published data on nearby stars and star clusters. The distances derived are 850 ± 300 pc for PK 128 −04 1 (S 188, Simeis 22), 400 ± 200 pc for PK 149 −09 1 (HW 4), and (less reliable) 800 pc ± 400 pc for PK 224 +01 1 (We 1–6). For the central star of PK 224 +01 1 our observations gave V = 15.76 ± 0.03 mag, (B–V) = −0.08 ± 0.03 mag and (U–B) = −0.87 ± 0.03 mag.