As the source star moves across the caustics, new images of the source can be created while others may disappear, resulting in changes of the image centroid. Colour curves show the path of the centroid of the source images relative to the unlensed position of the source (additional light from components of the lens is not included). Moments of Gaia transits are marked with black points. The coordinate system is the same as in Fig. 7. The shifts are scaled to the angular Einstein radius of the system (θ_E = 3.04 ± 0.24 mas). Analysis of the Gaia astrometric measurements will provide an independent estimate of θ_E.