UV observations with IUE have led to recognition of the importance of di-electronic recombination in PN, for the calculation of ionization equilibria and for production of the features C II λ1335 and C III λ2297.
The determination of C abundances is discussed. In high-excitation nebulae one must allow for absorption of C IV λ1549 by dust. It has been shown that C/O ≤ 0.5 for five PN with a silicate IR feature and that C/0 ≳ l for four with a SiC feature.
Dust opacities τD have been deduced from observed ratios (C IV λ1549)/(C III λ2297) and (C II λ1335)/(C I λ4267). The values obtained, τD ≃ 0.1, are consistent with the observed strength of thermal infra-red emission.
Earlier UV observations of PN were made by Pottasch et al. (1978) using ANS. Results obtained using IUE support neither their downward revisions of star temperatures nor their claim that a number of stars with TZ(He II) >> TZ (H I) are optically thick for H I.
From IUE observations it is concluded that the planetary κ 648 in M 15 has C/O = 2.4 and that its central star has a temperature of 30 000 κ and a luminosity of 1700 L⊙.