Logarithm of the gas surface density in the base model with the particle positions superimposed: 1 mm (red), 1 cm (green), 10 cm (orange). All particles are initially located beyond 20 a. The panels show the gap region, its surroundings (left), and the circumprimary disc (right). The smallest particles do not cross the gap region to reach the circumprimary disc, whereas the larger particles do. The secondary star is at apocentre. The colour bar represents Σ in units of M_A/a².