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H1504+65 is an extremely hot white dwarf (effective temperature Teff =200,000 K) with a carbon-oxygen dominated atmosphere devoid of hydrogen andhelium. This atmospheric composition was hitherto unique among hot white dwarfs(WDs), and it could be related to recently detected cooler WDs with C or Odominated spectra. The origin of the H and He deficiency in H1504+65 isunclear. To further assess this problem, we performed ultraviolet spectroscopywith the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope(HST). In accordance with previous far-ultraviolet spectroscopy performed withthe Far Ultraviolet Spectroscopic Explorer, the most prominent lines stem fromC IV, O V-VI, and Ne VI-VIII. Archival HST/COS spectra are utilized to provethat the supersoft X-ray source RX J0439.8-6809 is, considering the exoticcomposition, a twin of H1504+65 that is even hotter (Teff = 250,000 K). Incontrast to earlier claims, we find that the star is not located in the LargeMagellanic Cloud but a foreground object in the Galactic halo at a distance of9.2 kpc, 5.6 kpc below the Galactic plane, receding with vrad = +220 km/s.

Analysis of HST/COS spectra of the bare C-O stellar core H1504+65 and a high-velocity twin in the Galactic halo