= Roche lobe
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A <Roche lobe> is the region around one member of a circular, synchronously rotating <binary star> bounded by the critical <effective potential> surface through the <inner Lagrange point>. The two lobes meet at that saddle point. The construction includes the gravitational effects of both <stars> and the centrifugal contribution in the rotating frame; it is not simply a sphere at a fixed distance from one <star>.
A <star> that expands to fill this region can lose gas through <Roche-lobe overflow>. A <volume-equivalent radius> provides a convenient single size for the nonspherical lobe, allowing comparison with a model <stellar radius>. Changing the <binary mass ratio> or orbital separation changes that size, so orbital evolution can initiate transfer even without intrinsic stellar expansion.
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