We present a quantitative model of a proposed common slowly expanding envelope corotating with a symbiotic binary system. The geometry of the envelope is defined by the equipotential surface containing the Lagrangian pointL 2. The model was applied to the symbiotic binary EG And. The orbital inclination,i = 45°, and mass ratio of the cool to the hot component,q = 2.8–3.0, are determined. The nature of the primary as well as the secondary minima in the light curves, development of the line profiles, radial velocities and equivalent widths of the Hα line through an orbital revolution are explained by this model.