Chandra and FUSE spectroscopy of the hot bare stellar core H1504+65
  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 LETG spectrum (60-160 Angstroem) 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. 
K. Werner, T. Rauch, M. A. Barstow, J. W. Kruk