Resumen
Deep spectroscopic data obtained with the ESO Very Large Telescope, of planetary nebulae (PNe) and HII regions in the dwarf irregulars NGC 3109 and NGC 6822, are analyzed. We determined the physical conditions and the chemical composition in both type of nebulae. From HII regions we derived 12+log O/H=7.79+/-0.15 and 8.10+/-0.15 for NGC 3109 and NGC 6822 respectively, showing that NGC 3109 is one of the most metal-poor galaxies in the Local Group. Both galaxies appear very chemically homogeneous. Interestingly the PNe in NGC 3109 appear significantly O-richer than the HII regions, indicating that the central stars might have dredged-up some oxygen, enriching the stellar atmosphere previous to the PN ejection. Possibly the neon in the stellar atmospheres has been modified also. This is not the case for NGC 6822 where the PNe show in average the same abundances that HII regions.
