  In some semi-detached binary systems, the donor star may transfer mass to the
companion at a very high rate. We propose that, at sufficiently high
mass-transfer rates such that the accretion disk around the companion becomes
geometrically thick (or advection-dominated) near the disk outer radius, a
large fraction of the transferred mass will be lost through the outer
Lagrangian (L2) point, as a result of the excessive energy generated by viscous
heating that cannot be efficiently radiated away. A physical model is
constructed where the L2 mass loss fraction is given by the requirement that
the remaining material in the disk has Bernoulli number equal to the L2
potential energy. Our model predicts significant L2 mass loss at mass-transfer
rates exceeding a few times 10^{-4} Msun/yr. An equatorial circum-binary
outflow (CBO) is formed in these systems. Implications for the orbital
evolution and the observational appearance are discussed. In particular, (1)
rapid angular momentum loss from the system tends to shrink the orbit and hence
may increase the formation rate of mergers and gravitational-wave sources; (2)
photons from the hot disk wind are reprocessed by the CBO into longer
wavelength emission in the infrared bands, consistent with Spitzer observations
of some ultra-luminous X-ray sources.
