First row : three snapshots of one stream in the reference simulation, shown at the progenitor’s pericentre, apocentre, and at a time when the length asymmetry reaches one of its maxima. The black cross marks the Milky Way’s centre of mass and the blue line the progenitor’s trajectory. Second row : time evolution of the progenitor’s distance to the Milky Way’s centre of mass. Third row : time evolution of the length of the leading and trailing arm. Fourth row : normalised asymmetry between the leading and trailing arm lengths. The three vertical dotted lines mark the times of the snapshots shown in the top row. All time-dependent curves are smoothed with a 1D Gaussian filter with σ = 2.