BeppoSAX detected luminous 0.2–2.0 keV supersoft X-ray emission from the recurrent nova U Sco \sim 19-20 days after the peak of the optical outburst in February 1999 .
U Sco is the first recurrent nova to be observed during a luminous supersoft X-ray phase .
Non-LTE white dwarf atmosphere spectral models ( together with a \sim 0.5 keV optically thin thermal component ) were fitted to the BeppoSAX spectrum .
We find that the fit is acceptable assuming enriched He and an enhanced N/C ratio .
This implies that the CNO cycle was active during the outburst , in agreement with a thermonuclear runaway scenario .
The best-fit temperature is \sim 9 \times 10 ^ { 5 } K and the bolometric luminosity ( 0.16 - 1.2 ) \times 10 ^ { 36 } erg s ^ { -1 } ( d / kpc ) ^ { 2 } .
These values are in agreement with those predicted for steady nuclear burning on a WD close to the Chandrasekhar mass .
The fact that U Sco was detected as a supersoft X-ray source is consistent with steady nuclear burning continuing for at least one month after the outburst .
This means that only a fraction of the previously accreted H and He was ejected during the outburst and that the WD can grow in mass , ultimately reaching the Chandrasekhar limit .
This makes U Sco a candidate type Ia supernova progenitor .