  We present the X-ray spectrum of MR Vel/RXJ0925.7-4758 obtained with the
Medium Energy Grating spectrometer of the Chandra X-ray Telescope. The simplest
models used by earlier authors, stellar atmospheres in combination with a
thermal plasma in collisional ionization equilibrium, cannot explain the
spectrum. Neither does a photo-ionized plasma. We identify P Cygni profiles of
Fe XVII and O VIII, from which we conclude that these lines arise in a wind. We
conclude that major uncertainty exists about the bolometric luminosity of MR
Vel, and perhaps of supersoft sources in general, so that the theoretical
prediction that this luminosity derives from steady nuclear burning cannot be
verified.
