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We present deep spectrophotometry for a sample of 8 PNe and 12 HII regions inthe irregular galaxy NGC 3109, to analyze the chemical composition of bothtypes of nebulae. We present line intensities and the physical conditions andthe abundances of He, O, Ne, N, S and Ar are derived, using the classicalT_e-based method. We confirm our previous identification of PNe and HII regionsbased on photometry, except for one object that we argue is a compact HIIregion rather than a PN. We find that the chemical composition of the ISM inNGC 3109, as sampled by its HII regions, is remarkably uniform. The oxygenabundance is log O/H + 12 = 7.77 \pm 0.07 in this galaxy, as compared to 8.05\pm 0.09 for the SMC. PNe show significantly higher oxygen abundances in NGC3109: log O/H + 12 = 8.16 \pm 0.19. We argue that, similarly to what has beensuggested for some of the PNe in the Magellanic Clouds and other metal-poorgalaxies, oxygen in the PNe in NGC 3109 is affected by dredge up in theirprogenitors. This could be also the case for neon. From our analysis, weconclude that these two elements are not always a safe indicator of thechemical composition of the ISM at low metallicities. An alternative to the Oand Ne enrichment in PNe is that the low metallicity in HII regions has beencaused by dilution of the ISM due interaction with a neighbor galaxy about aGyr ago.  The excitation patterns of the PNe in NGC 3109 are very different from theexcitation patterns of PNe in other galaxies. This would imply that theevolution of PNe depends upon the properties of their progenitor stellarpopulations, which vary from galaxy to galaxy. This should affect the PNluminosity function and its use as a distance indicator.

The chemical composition of planetary nebulae and HII regions in NGC 3109