  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.
