  The gravity due to a multiple-mass system has a remarkable gravitational
effect: the extreme magnification of background light sources along extended
so-called caustic lines. This property has been the channel for some remarkable
astrophysical discoveries over the past decade, including the detection and
characterisation of extra-solar planets, the routine analysis of
limb-darkening, and, in one case, limits set on the apparent shape of a star
several kiloparsec distant. In this paper we investigate the properties of the
microlensing of close binary star systems. We show that in some cases it is
possible to detect flux from the Roche lobes of close binary stars. Such
observations could constrain models of close binary stellar systems.
