1 Introduction

Increasing use is being made of weak recombination lines to determine elemental abundances in photoionized nebulae relative to hydrogen (e.g. Liu et al. 1995). The most prominent lines of C, N, O and Ne arise from once ionized species for which reliable theoretical recombination coefficients are becoming available. Péquignot et al. (1991) have computed coefficients for selected lines of C, N and O in a relatively simple approximation, while more comprehensive and accurate calcaulations have been made by Davey et al. (1999) for CII, by Escalante & Victor (1990) for NII, by Storey (1994) for OII and by Kisielius et al. (1998) for NeII. For these once ionized species the transition arrays 4f - 3d, 3d - 3p and 3p - 3s fall in the blue part of the spectrum, mainly between 3500 and 5000 Å and are readily accessible to large ground-based telescopes. For twice ionized species, these transition arrays fall in the ultraviolet and are not easily observed, but the 5g - 4f transition arrays once again fall in the blue. In O²⁺, the 5g - 4f transitions lie between 4300 Å and 4600 Å and can be used to determine the O³⁺ abundance.

Péquignot & Baluteau (1994) have recently identified forbidden lines from elements with Z>30 in very deep red spectra of the planetary nebula NGC 7027. To be confident of the measured intensities of such lines it is necessary to understand the weak recombination lines, such as those of the 5g - 4f array, that are also present in the spectra with comparable intensity.