Surface density Σ for the simulation with 10 times smaller viscosity ν; the situationshort after the beginning at t = 100 P_orb (top panel), and the final state at t = 8000 P_orb (middle panel). The structures are initially more pronounced compared to the nominal case, because perturbations of the low-viscosity gas spread more slowly. The outcome is a massive co-orbital that migrates towards the inner boundary. The density contrast between Σ interior and exterior to the co-orbital is about 2; the disc is optically thick in both cases. The corresponding azimuthal velocity u_θ of pebbles (bottom) is compared to the Keplerian velocity v_kepl. Just outside the co-orbital pair, the dimension-less ratio (u_θ − v_kepl)∕v_kepl is positive (u_θ super-Keplerian) and a pebble isolation develops.