Roche-lobe overflow is mass transfer that begins when a donor star fills its Roche lobe. Gas near the inner Lagrange point can pass through the saddle in the rotating-frame effective potential toward the companion. Its orbital angular momentum can prevent direct radial infall and instead produce an accretion disk, depending on the geometry.
The process couples stellar evolution to orbital evolution. Losing mass changes the donor's stellar radius, while transferred or escaping matter changes the Roche lobe. Dynamical stability of binary mass transfer depends on which response dominates. Stable transfer can gradually strip an envelope; unstable interaction can lead to a common envelope. Case A mass transfer and Case B mass transfer distinguish the donor's nuclear state when overflow starts.
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