Method of calculations

The streamlines are symmetric with respect to the equatorial plane (the plane of the binary), and a streamline with angular momentum $l>l_\mathrm{min}$ will collide with the symmetric streamline coming from the other side of the equator. The trajectories of the streamlines are nearly parabolic. We will now derive an expression for the collision radius, for the Newtonian, Schwarzschild and Kerr cases.