A comparative study of planetary nebulae and HII regions in the same galaxy: the case of NGC 3109

After giant HII regions, which have been used for decades to determine metallicities and metallicity gradients in external galaxies, it is the turn of planetary nebulae - which are much fainter but can now be observed with large telescopes. The comparison of HII regions and planetary nebulae in the same galaxy leads to several surprises: we will present the results of our recent work on the galaxy NGC 3109, a galaxy of Magellanic type in the Local Group.