We have examined the optical/X-ray light curves of seven well-observed recurrent novae , V745 Sco , M31N 2008-12a , LMC N 1968 , U Sco , RS Oph , LMC N 2009a , T Pyx , and one recurrent nova candidate LMC N 2012a .
Six novae out of the eight show a simple relation that the duration of supersoft X-ray source ( SSS ) phase is 0.70 times the total duration of the outburst ( = X-ray turnoff time ) , i.e. , t _ { SSS } = 0.70 ~ { } t _ { off } , the total duration of which ranges from 10 days to 260 days .
These six recurrent novae show a broad rectangular X-ray light curve shape , first half a period of which is highly variable in the X-ray count rate .
The SSS phase corresponds also to an optical plateau phase that indicates a large accretion disk irradiated by a hydrogen-burning WD .
The other two recurrent novae , T Pyx and V745 Sco , show a narrow triangular shape of X-ray light curve without an optical plateau phase .
Their relations between t _ { SSS } and t _ { off } are rather different from the above six recurrent novae .
We also present theoretical SSS durations for recurrent novae with various WD masses and stellar metallicities ( Z = 0.004 , 0.01 , 0.02 , and 0.05 ) and compare with observed durations of these recurrent novae .
We show that the SSS duration is a good indicator of the WD mass in the recurrent novae with a broad rectangular X-ray light curve shape .