From the summary: The envelope of iso-energetic trajectories in the (repulsive) two-fixed- centre problem is derived. Our analytical calculations finally lead to a transcendental equation, only containing elliptic integrals and the Weierstraß \(\wp\) function, from which the envelope is constructed. The results may serve as a simple model for the boundary layer between two colliding supersonic stellar wind flows in binary systems, in which at least one of the components has a strong radiation field. Beyond this, the effect of non-inertial forces (centrifugal and Coriolis force) due to the binary’s orbital motion has been estimated by a numerical analysis within the scope of the (repulsive) restricted three-body problem.