  Images obtained with the ESO VLT and FORS1 in [O III] 5007 on- and off-band
as well as r_Gunn filters are analyzed to search for Planetary Nebula (PN)
candidates in the dwarf irregular NGC 3109. In the continuum-subtracted [O III]
5007 on-band images, a large number of emission-line regions were detected. We
describe the criteria employed in order to distinguish PN candidates from
compact HII regions, finding that the sizes and the properties of the ionizing
stars are the most unambiguous discriminators for separating the two classes.
Based upon these criteria, we have found 20 PN candidates for which we present
coordinates, nebular [O III] fluxes, and stellar magnitudes. The cumulative
luminosity function for these PNe is discussed. A catalog of HII regions
listing coordinates, nebular [O III] fluxes, stellar magnitudes and other
characteristics is also presented. We find that HII regions are rather
concentrated towards the disk of the galaxy, while PNe are found also above and
below this structure, consistent with their belonging to an older stellar
population.
