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Planetary nebula abundances in dwarf galaxies

Abstract The framework of the present review is as follows. The Local Group (LG) is populated by dwarf galaxies belonging to different morphological types. Apparently these galaxies are very different. The questions naturally raised from this framework are many. Is there an evolutionary sequence among them? Do they share common progenitors? Is the environment at the origin of their differences? The method we propose to answer to these questions is by investigating the chemical evolution of dwarf galaxies and their mass-metallicity relation (MZR). To this aim we use metallicities derived from planetary nebulae, since this stellar population is present in the star-forming (dwarf irregular, dIrr) as well as in the quiescent (dwarf spheroidal, dSph) galaxies. The results, actually, show that both dIrr and dSph galaxies of the LG follow the same MZR, at variance with the differences claimed in the past. These results are in good agreement with the recently derived MZR, based on stellar instead of the nebular metallicities of the LG dwarf galaxies. Moreover, our MZR is also consistent with the global MZR of SLOAN star-forming galaxies, which spans a wider stellar mass range (~ 106 - 1011 M⊙) than the LG dwarfs.