Schematic diagram showing the interaction of two spherical winds that move radially away from the stars. Source 1 of the wind with velocity v₁ is located at the origin of the coordinate system, and source 2 of the wind with velocity v₂ at a distance D along the symmetry axis. The shape of the layer where the winds collide (at R₀ along the symmetry axis) is given by the curve R(θ). The angles θ and θ₁ are measured from the positions of the stars to the intersection point between a line of sight (with impact parameter r) and the layer. We assume azimuthal symmetry (no dependence on angle φ). The observer is located in the orbital plane along the z-axis of symmetry.