A Roche lobe is the region around one component of a circular binary inside the critical effective-potential surface through the inner Lagrange point.
Conservative binary mass transfer moves matter between the components while preserving total binary mass and orbital angular momentum.
Binary mass transfer is dynamically stable when the donor's radius decreases relative to its Roche-lobe radius after a small mass loss, reducing overflow instead of amplifying it.
A common-envelope phase occurs when unstable binary interaction engulfs both stellar cores in one envelope. Orbital energy and angular momentum may eject the envelope, leaving a compact binary, or the cores may merge.
