Top: cross-correlation map obtained by comparing two brightness maps obtained using 2018 data from 18 Oct., versus 23 Oct. (with a gap of 15.6 rotation periods between the two maps). Bottom: same, but for the two observing nights of our 2006 data (18.7 rotation periods apart). The blue lines show the differential rotational law derived using the sheared imaged method in Sect. 4 (top panel), and in C20 (bottom panel). The black, dashed line marks a null phase shift (which is equivalent to a solid-body rotation).