An artist's concept of a close Algol-type binary. The
relative size of the Sun is illustrated by the small circle to the upper right
of the figure. Illustration courtesy of M. Richards.
In Algol-type binaries, one of the stars has evolved and expanded to fill a droplet-shaped potential surface, called the Roche lobe, within which material is gravitationally bound to the star (see Figure 1). The Roche surface is, therefore, the surface along which the gravitational potential is common between the stars. Once a star fills its Roche lobe, gas moves into the Roche lobe of the companion star and is pulled in toward that star. This process of mass transfer is referred to as Roche lobe overflow. Binaries in this stage of mass transfer are called semi-detached binaries, because only one of the stars is actually in contact with its Roche surface. The subsequent flow of gas between the stars is called the gas stream or mass transfer stream. During Roche lobe overflow, mass transfer feeds gas particles in the stream from the inner Lagrangian point (L1), where the two Roche lobes touch. This gas stream free-falls onto the companion star, much like rocks dropped from a building. However, the path of the gas stream becomes curved because it feels the orbital motion of the binary (or the Coriolis force) as it falls.