Roche Lobe

The Roche lobe is a volume around a star in a binary system, inside which material is bound to that star. At the surface of the Roche lobe, counteracting gravitational forces due to both stars effectively cancel each other out. In other words, the Roche lobe surface is a gravitational equipotential surface.

The red line in the following figure represents the equipotential surface of the Roche lobe in a HMXB (left) and LMXB (right).

Figure 3 from The Hydrodynamics of Accretion from Stellar Winds, Ronald E. Taam & Bruce A. Fryxell, American Scientist, November-December 1989, pp. 539-545.