5 Conclusions

We have shown that:

• Nuclear H II regions, Seyfert 2s and Liners lie in distinct, well separated regions in the log($\lambda 5007/{\rm H}\beta$) vs. log($\lambda\,6583$/$\rm H\alpha$) and log($\lambda 5007/{\rm H}\beta$) vs. log($\lambda 6300/{\rm H}\alpha$) diagrams. There is no continuity between Liners and Seyfert 2s, with an apparent deficit of objects with $\lambda 6300/\lambda 5007$ = 0.25.
• A number of objects have "transition'' spectra, falling outside the regions assigned to the three types of emission nebulosities. They probably all have a "composite'' spectrum.
• We have isolated a class of Seyfert 2 galaxies with weak [N II] lines. This weakness could be due to an under-abundance of nitrogen or to the presence of matter-bounded clouds in these objects.

Acknowledgements

This research has made use of the NASA/IPAC extragalactic database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration. A.C. Gonçalves acknowledges support from the Fundação para a Ciência e a Tecnologia, Portugal, during the course of this work (PhD. grant Ref. PRAXIS XXI/BD/5117/95).