The region surrounding each star in a binary system, within which any material is gravitationally bound to that particular star. The boundary of the Roche lobes is an equipotential surface, and the lobes touch at the inner Lagrangian point, L₁, through which mass transfer may occur if one of the components expands to fill its lobe. It is named after the French mathematician Édouard Albert Roche (1820–83).

Roche lobe:

In a binary system, the Roche lobes of components A and B meet at the L₁ Lagrangian point. (a) In a detached system, neither star fills its Roche lobe. (b) In a semidetached system, one massive component, B, fills its Roche lobe. (c) In a contact binary both components overfill their Roche lobes and share a common envelope.