H1504+65 is an extremely hot white dwarf ( effective temperature T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 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 utilised to prove that , considering its exotic composition , the supersoft X-ray source RX J0439.8 $ - $ 6809 is an even hotter ( T \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 250 000 K ) twin of H1504+65 .
In contrast to earlier claims , we find that the star is not located in the Large Magellanic Cloud , but is a foreground object in the Galactic halo at a distance of 9.2 kpc , 5.6 kpc below the Galactic plane , receding with v _ { rad } = +220 km s ^ { -1 } .