Location of spectroscopically confirmed PNe (filled red points), PNe candidates (filled orange points) and H II regions (open blue circles) in a diagram of [O III] λλ4959,5007/Hβ versus [S II] λλ6716,6731/[O III] λλ4959,5007. We applied a first order background correction to the PN fluxes as described in Sect. 3.3. We observe that the confirmed PNe occupy a well-defined region in the diagram.