Equilibrium configurations of the spin axes. These correspond to directions that are either inside the orbital plane, in some arbitrary direction (case 1, left column), or orthogonal to the orbital plane (case 2, right column). In each case, the spins may either be parallel (top) or anti-parallel (bottom). As discussed in Sect. 4.2, only the anti-parallel orthogonal configuration (bottom-right) is the only stable scenario.