= Roche-lobe overflow
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<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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