A donor star is the component of a binary star that supplies matter to its companion. In Roche-lobe overflow, it fills its Roche lobe and gas passes near the inner Lagrange point. In wind mass transfer in a binary star, some gas lost in a stellar wind is captured by the other component without the donor necessarily filling its Roche lobe.
The donor's response to losing mass determines whether transfer weakens or strengthens. Compare its stellar radius response exponent with the Roche-lobe radius response exponent: if mass loss increases the relative overfill, transfer can run away. The donor need not currently be the more massive member, because binary mass-ratio reversal can occur after substantial transfer.
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