Recently, Kitamura and Nakamura have found that the anomalous gravity darkening occurs in semi-detached binary systems. The exponent of gravity darkening for the secondary components, which is defined by where F is the radiative flux and g is the gravitational accerelation, is significantly greater than the unity as shown in Table 1. We interpret this in terms of the energy transport by the mass outflow from the secondary component filling the Roche-lobe.

Table 1.

Empirical αc-values and physical quantities of five semi-detached binary systems.

From the analysis based on this interpretation, we show that 1) the mass out-flow carries energy if ∇ < ∇ad and 2) the anomalous gravity darkening requires that the flow must originate from deep interior. The mass loss rate can be estimated from the anomalous gravity darkening. Thus, the anomalous gravity darkening can be used not only to estimate the mass transfer rate but also to probe the interior structure and the evolution of the secondary star of the semi-detached binary. This model links Morton’s instability[2] of the secondary with the accretion disk of the primary.