  We have searched for the emission from \fex and \fexiv that is expected from
the gas emitting in diffuse X-rays in \bd, NGC 6543, NGC 7009, and NGC 7027.
Neither line was detected in any object. Models that fit the X-ray spectra of
these objects indicate that the \fex emission should be below our detection
thresholds, but the predicted \fexiv emission exceeds our observed upper limits
(one sigma) by factors of at least 3.5 to 12. The best explanation for the
absence of \fexiv is that the X-ray plasma is depleted in iron. In principle,
this result provides a clear chemical signature that may be used to determine
the origin of the X-ray gas in either the nebular gas or the stellar wind. At
present, though various lines of evidence appear to favour a nebular origin,
the lack of atmospheric and nebular iron abundances in the objects studied here
precludes a definitive conclusion.
