Ratio of the tangential to radial component of the radiative force in the plane of stellar centres and rotational axis. Mass ratio q = 2 / 3, volume of the Roche equipotential (drawn as a light blue line) 0.95 of the critical Roche lobe. Limb- and gravity-darkening (0.0, 0.0) and (0.6, 0.25) are plotted in the left- and right-hand panel. The black isocontours correspond to the values of − 3, − 2, and − 1.