Total angular momentum of the system L_tot (in red) as resulting from the composition of the stellar spin angular momentum I_s Ω_s (in green) and the orbital angular momentum l_orb (in blue). The angles ϵ_s and ϵ_orb between the stellar spin or the orbital angular momentum and the total angular momentum, respectively, are indicated. The obliquity of the system is ϵ = ϵ_s + ϵ_orb, while its projection on the plane of the sky is λ (not indicated).