  The highly luminous (> 10^37 erg s^-1) supersoft X-ray sources (SSS) are
believed to be Eddington limited accreting white dwarfs undergoing surface
hydrogen burning. The current paradigm for SSS involves thermally unstable mass
transfer from a 1-2 solar mass companion. However this model has never been
directly confirmed and yet is crucial for the evolution of cataclysmic
variables in general, and for the establishment of SSS as progenitors of type
Ia supernovae in particular. The key SSS is RX J0513.9-6951 which has recurrent
X-ray outbursts every 100-200 d (lasting for ~40 d) during which the optical
declines by 1 mag. We present the first XMM-Newton observations of RX
J0513.9-6951 through one of its optical low states. Our results show that as
the optical low state progresses the temperature and the X-ray luminosity
decrease, behaviour that is anti-correlated with the optical and UV emission.
We find that as the optical (and UV) intensity recover the radius implied by
the spectral fits increases. The high resolution spectra show evidence of deep
absorption features which vary during the optical low state. Our results are
consistent with the predictions of the white dwarf photospheric contraction
model proposed by Southwell et al. 1996.
