A star with mass M _⋆  is perturbed by a binary system with masses M₁ and M₂. The Jacobi coordinates r_b and r are, respectively, the inter-binary distance and the distance of M _⋆  to the binary’s center of mass. The unperturbed Keplerian described by r is in the plane (x′,y′). The unperturbed Keplerian described by r_b is defined with respect to the (x′,y′) plane by the angles Ω (relative node longitude) and i (relative inclination). The observer’s frame (x,y,z) with z being the line of sight is obtained by rotating the auxiliary frame (x′,y′,z′) around the x axis by an angle I (inclination of r’s orbit with respect to (x,y)). Moreover, S = ∠(r,r_b), , and .