Orbit frame (x_orb, y_orb, z_orb) and orbital elements are represented in purple, while the fundamental frame (X, Y, Z) is in green. The pericenter, represented by the point 𝒫, is also added. Finally, ℒ_orb//sky denotes the line of nodes of the orbit frame and sky plane. We also represent the Gaussian frame of the star (n_orb, m_orb, z_orb), the projection of the X axis on the orbit plane denoted by X_//orb and the angle, ϖ, introduced in Sect. 4. Here, we illustrate the fundamental frame when it matches that of a distant observer (see Sect. 2). Also, it is useful to note the the azimuthal angle is φ = ω + f.