  H1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective
temperature close to 200,000 K. We present new FUV and soft X-ray spectra
obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere
primarily composed of carbon and oxygen. The Chandra spectra show a wealth of
absorption lines from highly ionized oxygen, neon and magnesium and suggest
relatively high Ne and Mg abundances. This corroborates an earlier suggestion
that H1504+65 represents a naked C/O stellar core or even the C/O envelope of a
O-Ne-Mg white dwarf.
