One of the candidates for Type la supernova progenitors, the recurrent nova RS Ophiuchi, underwent its sixth recorded outburst in 2006 February, and for the first time a complete light curve of supersoft X-rays has been obtained. It shows the much earlier emergence and longer duration of a supersoft X-ray phase than expected before. These characteristics can be naturally understood when a significant amount of helium layer piles up beneath the hydrogen-burning zone during the outburst, suggesting that the white dwarf (WD) is effectively growing in mass. We have estimated the WD mass in RS Oph to be 1.35 ±0.01 M ⊙ and the growth rate of the WD mass to be at an average rate of ∼1 × 10-7 M⊙ yr-1. The white dwarf will probably reach the critical mass for Type la explosion if the present accretion continues further for a few to several times 105 yr.
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