SIMBAD references

A diagnostic diagram for separating planetary nebulae from symbiotic stars, based only on the relation between the [O III] lines at 5007 and 4363 A, is presented. It separates very well the two types of objects, as well as S- and D-type symbiotics. There is an intermediate region in which very young planetary nebulae seem to be located. The behavior of electron temperatures and densities in the diagram is studied. An analysis of some objects shows that many D-type symbiotics move in their region and also from D-type region towards that of young planetary nebulae, in such a way that it becomes difficult to separate these two kinds of objects.