Reference frame adopted to specify the geometry of the WASP-33 system. The origin O is at the barycentre of the system; the axis is directed along the total angular momentum of the system (in red); and the axis is in the plane defined by the line of sight and the total angular momentum. The vector (dashed in green) is directed opposite the line of sight, meaning from the barycentre of the system towards the observer on Earth. The unit vector in the direction of the orbital angular momentum is depicted in blue (solid line), together with its projection I_xy on the xy plane (dashed line), and the angle α_orb between that projection and the axis. The angle α_orb varies uniformly in our inertial reference frame due to the precession of the orbital plane (see the text). For simplicity, the stellar spin vector and its projection on the xy plane are not shown.