H1504+65 - The Naked Stellar C/O Core of a Former Red Giant Observed   with FUSE and Chandra
  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. 
K. Werner, T. Rauch, M.A. Barstow, J.W. Kruk