Eggleton Roche-lobe radius approximation ($R_L/a=0.49q^{2/3}/[0.6q^{2/3}+\ln (1+q^{1/3})]$)

For a circular synchronized binary star, this approximates the volume-equivalent Roche lobe radius, where $q$ is the enclosed star's mass divided by its companion's mass. It describes critical equipotential geometry; it does not give the star's radius response to mass loss. The primary calculation reports radius errors below about one percent across the mass-ratio range.^[ref]