Scheme of a binary system separated by a distance r. The different quantities used to model the deformations in the perturbative framework are detailed and illustrated here. The blue curve is the surface of the deformed star while the red curves are the spheres of equivalent averaged radii. As the surface is represented here, r̅_i(μ_i,ϕ_i) = R_i, R_i being the radial coordinate of the stellar surface in the spherically symmetric case.