The Nature of the Luminous Supersoft X-ray Sources

The highly luminous (>10e37 erg/s) supersoft Xray sources (SSS) are believed to be superEddington limited accreting white dwarfs undergoing surface hydrogenburning. The current paradigm for SSS (van den Heuvel et al. 1992) involves thermally unstable mass transfer from a 12 Msun companion. But this model hasnever been directly confirmed and yet is crucial for the evolution of cataclysmic variables in general, and for the establishment of SSS as progenitors oftype Ia supernovae in particular. The key SSS is RXJ0513.96951 which hasrecurrent Xray outbursts every 100150d (lasting virgul30d) during which the opticaldeclines by 1 mag. We request TOO status to be triggered by optical monitoringso as to follow directly the predicted expansion of the white dwarf photosphere. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]