The discovery of photospheric nickel in the hot DO white dwarf REJ 0503-289

We present the first evidence for the direct detection of nickel in the pho tosphere of the hot DO white dwarf REJ 0503-289, While this element has bee n seen previously in the atmospheres of hot H-rich white dwarfs, this is on e 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 an Fe/ Ni ratio a factor of 10 lower than seen in the H-rich objects (Ni/He = 10(- 5) for REJ 0503-289), The abundances of nickel and various other elements h eavier than He were determined from Goddard High Resolution Spectrograph sp ectra, We used two completely independent sets of non-local thermodynamic e quilibrium 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-eff and log g, from the He lines in the optical spectrum, in the form of a formal fitt ing of the line profiles to a grid of model spectra, an analogue of the sta ndard procedure utilizing the Balmer lines in DA white dwarfs. This gives t he assigned uncertainties in T-eff and log g a firm statistical basis and a llows us to demonstrate that inclusion of elements heavier than H, He and C in the spectral calculations, exclusively considered in most published opt ical analyses, yields a systematic downward shift in the measured value of T-eff.