  We present the first evidence for the direct detection of nickel in the
photosphere of the hot DO white dwarf REJ0503$-$289. While this element has
been seen previously in the atmospheres of hot H-rich white dwarfs, this is one
of the first similar discoveries in a He-rich object. Intriguingly, iron, which
is observed to be more abundant than Ni in the hot DA stars, is not detected,
the upper limit to its abundance (Fe/He$=10^{-6}$) implying a Fe/Ni ratio a
factor 10 lower than seen in the H-rich objects (Ni/He$=10^{-5}$ for
REJ0503$-$289). The abundance of nickel and various other elements heavier than
He were determined from GHRS spectra. We used two completely independent sets
of NLTE model atmospheres which both provide the same results. This not only
reduces the possibility of systematic errors in our analysis but is also an
important consistency check for both model atmosphere codes.
  We have also developed a more objective method of determining $T_{\rm eff}$
and log g, from the He lines in the optical spectrum, in the form of a formal
fitting of the line profiles to a grid of model spectra, an analogue of the
standard procedure utilising the Balmer lines in DA white dwarfs. This gives
the assigned uncertainties in $T_{\rm eff}$ and log g a firm statistical basis
and allows us to demonstrate that inclusion of elements heavier than H, He and
C in the spectral calculations, exclusively considered in most published
optical analyses, yields a systematic downward shift in the measured value of
$T_{\rm eff}$.
