Evolution of the radius (top) and phase (bottom) of the global maximum of the effective potential Eq. (12) as a function of the mass of the central bar. For smaller values of the bar’s mass, the equilibrium solution is associated to the stable L₄-centre of corotation. Its monotonous evolution with the increasing mass is interrupted at M_bar = 2.6 × 10¹⁰M_⊙, when L₄-centre becomes unstable and B₁-centre has maximal energy (see Fig. 5).