Definition of the azimutal angle θ. The plane depicted is the plane of the binary orbit; the binary orbit is depicted in orange. The two stars are symbolised by the two orange dots. The r, l, and n are the rotation axis of the star, the line of sight, and the normal to the orbital plane, respectively, while the angles i and α are, respectively, the orbital inclination and the angle between the stellar rotation axis and the normal to the orbital plane.