Abundances and morphology in planetary nebulae

The abundances of 16 well studied have been determined. New ISO measurements have been combined with optical and ultraviolet data from the literature, in an attempt to obtain accurate values. Only He, O, C, N, Ne, Ar, and S are considered. High values of N/O are sometimes, but not always, found in bipolar nebulae. On the other hand, some bipolar nebulae show low values of N/O, and it is concluded that no simple relationship between morphology and composition exists.