Possible spin-orbit configurations. The astrometric motion in the sky plane defined by the north and east axes is observed from location P. The orbit orientation vector (red arrow) is defined by the angles i_orbit and Ω. The stellar spin orientation vector can lie on either the dark-grey cone (prograde configurations) or the light-grey cone (retrograde configurations) with identical opening angles 2 × (180° − i_rot) = 51°.