We present a dual abundance photoionization model of the SMC planetary nebula (PN) N87, based on NTT 3.5-m spectroscopy of faint optical recombination lines (ORLs) of carbon and oxygen, along the principles laid out by Péquignot et al. (2002) in their modelling of the galactic PN NGC 6153. We show that the factor of ~2-3 discrepancy for the abundances of carbon and oxygen derived empirically from ORLs as opposed to those derived from forbidden lines can be explained by introducing chemical inhomogeneities in the nebula in the form of hydrogen-deficient, cold plasma regions. Interestingly, in contrast to similar studies thus far, we find the C/O elemental abundance ratio in the H-deficient regions to be significantly lower that the same abundance ratio in the 'normal' composition nebular gas. This finding raises important questions regarding the relative chemical history of the two main constituent components of the nebula.

This is the first time, to our knowledge, that a self-consistent, dual abundance photoionization model has been constructed for an extragalactic PN.