2004A&A...421.1169W

object{H1504+65} is an extremely hot hydrogen-deficient white dwarf with an effective temperature close to 200000K. 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 LETG spectrum (60-160Å) shows a wealth of photospheric absorption lines from highly ionized oxygen, neon, and - for the first time identified in this star - magnesium and suggests 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 an O-Ne-Mg white dwarf.