  We have detected the [S IV] 10.5 micron and [Ne II] 12.8 micron
fine-structure lines in the halo population planetary nebula (PN) DdDm 1, and
set upper limits on their intensities in the halo PN H 4-1. We also present new
measurements of optical lines from various ions of S, Ne, O, and H for DdDm 1,
based on a high-dispersion spectrum covering the spectral range 3800 A to 1
micron. These nebulae have similar O/H abundances, (O/H) = 1e-4, but S/H and
Ne/H are about half an order of magnitude lower in H 4-1 than in DdDm 1; thus H
4-1 appears to belong to a more metal-poor population. This supports previous
suggestions that PNe arising from metal-poor progenitor stars can have elevated
oxygen abundances due to internal nucleosynthesis and convective dredge-up. It
is generally accepted that high abundances of carbon in many PNe results from
self-enrichment. To the extent that oxygen can also be affected, the use of
nebular O/H values to infer the overall metallicity of a parent stellar
population (for example, in external galaxies) may be suspect, particularly for
low metallicities.
