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 { Ly \alpha } absorption line is consistent with an interstellar column density of neutral hydrogen \mbox { $N _ { HI } $ } = ( 4.0 \pm 1.0 ) \times 10 ^ { 20 } cm ^ { -2 } .
The light curve obtained from the time-tagged dataset puts a 3 \sigma upper limit of 0.04 mag on the ultraviolet variability of RX J0439.8 - 6809 on time scales between 10 s and 35 min .
The long-term X-ray light curve obtained from our three-year ROSAT HRI 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 1000 K. 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 .