H1504+65 is an extremely hot white dwarf (effective temperature Teff = 200 000 K) with a carbon-oxygen dominated atmosphere devoid of hydrogen and helium. This atmospheric composition was hitherto unique among hot white dwarfs (WDs), and it could be related to recently detected cooler WDs with C or O dominated spectra. The origin of the H and He deficiency in H1504+65 is unclear. To further assess this problem, we performed ultraviolet spectroscopy with the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST). In accordance with previous far-ultraviolet spectroscopy performed with the Far Ultraviolet Spectroscopic Explorer, the most prominent lines stem from C iv, O v-vi, and Ne vi-viii. Archival HST/COS spectra are utilized to prove that the supersoft X-ray source RX J0439.8-6809 is, considering the exotic composition, a twin of H1504+65 that is even hotter (Teff = 250 000 K). In contrast to earlier claims, we find that the star is not located in the Large Magellanic Cloud but a foreground object in the Galactic halo at a distance of 9.2 kpc, 5.6 kpc below the Galactic plane, receding with vrad = +220 km s-1.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26666.