Final answer:
In an X-ray binary system, mass transfer occurs when the companion star fills its Roche Lobe and its matter gets transferred to the compact star.
Step-by-step explanation:
In an X-ray binary system, if the companion star fills its Roche Lobe, its matter will be transferred to the compact star through mass transfer.
An X-ray binary system consists of a compact object, such as a black hole or a neutron star, and a companion star. When the companion star fills its Roche Lobe, which is its gravitational boundary, the matter from the companion star gets pulled towards the compact star due to gravity. This transfer of matter from the companion star to the compact star is known as mass transfer.