The PN candidates in NGC 3109: VLT-FORS1 imaging and spectroscopy

From VLT FORS1 on-band, off-band imaging of the dIrr NGC3109 we have identified about 13 PN candidates. According to our criteria for PN candidate selection, most of the candidates reported by Richer & McCall (1992) are in fact compact HII regions. Further multi-object VLT FORS1 spectroscopy has confirmed the PN nature of at least 6 candidates. All but one of the PNe analyzed are low excitation nebulae, showing no He II emission lines. For several PNe and HII regions, we derived chemical abundances based on electron temperatures measured from the [O III] 4363/5007 line ratios. Preliminary results show that PNe present log(O/H)+12 in the range from 7.7 to 8.4, while in HII regions this quantity spreads in a much narrower range from 7.6 to 7.8. Thus, the ISM in NGC 3109 seems chemically homogeneous.