SIMBAD references

The question as to the hydrogen content in the atmospheres of helium-rich white dwarfs is of considerable importance to explain the long-standing problem of the chemical surface abundance evolution of white dwarfs along their cooling sequence. It is hoped that the H abundance gives a hint to the origin of the bifurcation of the white dwarf cooling sequence into a H-rich (DA) and a He-rich (non-DA) branch and the possible relationship between these. We present high resolution spectra from two hot He-rich white dwarfs (RE0503-289 and PG1034+001) and the prototype of the PG1159 spectral class (PG1159-035). The spectra cover the core of the He II 6560A line and were taken in order to detect possible weak Hα emission components. The search was entirely negative and imposes an upper limit (≃5% by number) for the hydrogen abundance which is tighter than determined from previous medium resolution spectroscopy. We consider implications of this result on the evolutionary scenario.