1999A&A...351L..27V

We present ultraviolet observations of the supersoft X-ray source RX J0439.8-6809 obtained with the Hubble Space Telescope Imaging Spectrograph. The ultraviolet spectrum is a very blue continuum overlayed with interstellar absorption lines. The observed broad Lyα absorption line is consistent with an interstellar column density of neutral hydrogen N_HI=(4.0±1.0)x10²⁰cm^–2. The light curve obtained from the time-tagged dataset puts a 3σ upper limit of 0.04mag on the ultraviolet variability of RX J0439.8-6809 on time scales between 10s and 35min. The long-term X-ray light curve obtained from our three-year ROSATHRI monitoring of RX J0439.8-6809 shows the source with a constant count rate, and implies that the temperature did not change more than a few 1000K. If RX J0439.8-6809 is a massive extremely hot pre-white dwarf on the horizontal shell-burning track, opposed to the alternative possibility of a very compact double-degenerate supersoft X-ray binary, its constant temperature and luminosity are a challenge to stellar evolution theory. Interestingly, RX J0439-6809 is found close to the theoretical carbon-burning main-sequence.