2015A&A...584A..19W

object{H1504+65} is an extremely hot white dwarf (effective temperature T_eff=200000K) 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 CIV, OV-VI, and NeVI-VIII. Archival HST/COS spectra are utilized to prove that the supersoft X-ray source RXJ0439.8-6809 is, considering the exotic composition, a twin of H1504+65 that is even hotter (T_eff=250000K). 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 9.2kpc, 5.6kpc below the Galactic plane, receding with v_rad=+220km/s.