OXYZ reference frame with its origin in the barycentre O of the star, the Z axis along the stellar spin axis Ω_⋆, and the XZ plane oriented as to contain the line of sight . The inclination of the stellar spin axis to the line of sight is indicated with i. The red vector indicates the projection of the rotation axis over the plane of the sky, that is the plane xy of the reference frame Oxyz, obtained from OXYZ by making a rotation of the angle i along the Y axis. We note that the projection of the spin axis onto the xy plane is opposite to the positive direction of the x axis because the angle i is taken positive (counterclockwise) from the spin axis to the line of sight.