We consider the effects of mutual transits by extrasolar planet-companion systems ( in a true binary or a planet-satellite system ) on light curves .
We show that induced changes in light curves depend strongly on a ratio between a planet-companion ’ s orbital velocity around their host star and a planet-companion ’ s spin speed around their common center of mass .
In both the slow and fast spin cases ( corresponding to long and short distances between them , respectively ) , a certain asymmetry appears in light curves .
We show that , especially in the case of short distances , occultation of one faint object by the other , while the transit of the planet-companion system occurs in front of its parent star , causes an apparent increase in light curves and characteristic fluctuations appear as important evidence of mutual transits .
We show also that extrasolar mutual transits provide a complementary method of measuring the radii of two transiting objects , their separation and mass , and consequently identifying them as a true binary , planet-satellite system or others .
Monitoring 10 ^ { 5 } stars for three years with Kepler may lead to a discovery of a second Earth-Moon-like system if the fraction of such systems for an averaged star is larger than 0.05 , or it may put upper limits on the fraction as f < 0.05 .