Tidal enhancement by a binary companion of stellar winds from cool giants.

Certain RS CVn binaries show 'mass inversion' (i.e. the more evolved star being the less massive) before Roche lobe overflow (RLOF) has taken place. It is found that the mass loss rates of ordinary giants are too small by a large factor to produce the observed masses of Z Her, the best example of this phenomenon. It is postulated that the originally less massive secondary has tidally enhanced the rate of mass loss by stellar wind from the primary, and a simple model of this enhancement is investigated over a wide range of initial mass ratios and periods. It is found that the period/mass ratio space divides into three regions: (1) no Roche lobe overflow occurs before a white dwarf is produced leading to systems like FF Aqr and AY Cet; (2) RLOF occurs after the mass ratio q has been reduced from q(0) of greater than 1 to q of less than 0.7, after which slow (nuclear time-scale) mass transfer takes place allowing Algols to be formed from late Case B systems without drastic mass transfer; and (3) RLOF occurs while q is greater than 0.7 resulting in rapid (hydrodynamic time-scale) mass transfer, probably leading to common envelope evolution and possibly to coalescence.