Location of the equilibrium radius r_eq in pc (contours) of momentum coupled grains at t = 3 Myr (present situation), evaluated for a grid of radiation pressure efficiencies and ratios of geometrical cross-section to the mass of the grain, ξ. The thick black contours represent r_eq = 0.1 and r_eq = 1.0, respectively, corresponding to the peak intensities of the inner dust wave (IDW) and outer dust wave (ODW). The light-shaded areas mark the boundaries in projected distance at which the dust waves are observed Fig. 3. The black area represent grains that overshoot r_eq to reach r_min, and are thus not momentum coupled. The vertical axis on the right denotes the grain radius if the grain mass is distributed in a compact geometry.