One of the candidates for Type Ia supernova progenitors , the recurrent nova RS Ophiuchi underwent the sixth recorded outburst in February 2006 , and for the first time a complete light curve of supersoft X-ray 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 up in mass .
We have estimated the WD mass in RS Oph to be 1.35 \pm 0.01 ~ { } M _ { \odot } and the growth rate of the WD mass to be at an average rate of about 1 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } .
The white dwarf will probably reach the critical mass for Type Ia explosion if the present accretion continues further for a few to several times 10 ^ { 5 } years .