Surface density of the disc at different times of the nominal simulation. The filled and empty circles mark the positions of Jupiter and Saturn, respectively. Panel a shows the disc at the moment of introduction of Saturn, T_0‚S, while the outer planet still has zero mass. In panel b, the planet is at 45% of its total mass. In panel c, the planet is just about to reach its final mass. Panel d shows the disc and the planets closer to the end of the simulation. At this point both planets are in a large common gap and locked in the 2:1 MMR. The colours given to the planets correspond to the markers in Fig. 3.