Radii (in gravitational radii, r_g = GM/c²) of the innermost stable circular orbit, r_isco, as a function of the spin parameter, a. The black thick line corresponds to the non-magnetised case (which, at first order in λ, remains unchanged for the uniform magnetic field case). Dashed lines correspond to first-order correction to r_isco for the dipolar magnetic field configuration for different values of λ (see the legend).