The analysis of emission line spectra of photoionized nebulae allows us to determine the chemical composition of the interstellar medium from the solar neighbourhood to the high-redshift star-forming galaxies. It stands as an essential tool for our knowledge of the nucleosynthesis and cosmic chemical evolution.

The advent of the large aperture (10m-class) telescopes and the future installation of giant-class ones (diameters 30-50 m) opens new horizons in the field of nebular spectroscopy. The detection of very faint emission lines in ionized nebulae as auroral collisionally excited lines (hereinafter CELs) in faint, distant or high-metallicity objects; recombination lines (hereinafter RLs) of heavy-element ions or CELs of trans-iron s-elements is becoming a routine fact and provide new information of paramount interest in many different areas of astrophysics.

The measurement and analysis of these kinds of lines and the implications of the chemical abundances determined from them are the main objectives of this research project, that can be summarized in the following ones:

a) to explore the evolution of C in galaxies and its nucleosynthesis in stars of different masses using abundances determined from RLs in extragalactic HII regions in spiral galaxies and the Magellanic Clouds (MCs), double chemistry dust planetary nebulae (PNe) and ejecta ring nebulae around massive stars;

b) to study the properties of dust depletion in the MCs through the determination of Fe abundances in HII regions and PNe;

c) to determine and analyse s-elements abundance ratios and their correlation with C/O ratios in PNe.