Top panel: ESPRESSO radial velocities of the WASP-121b transits observed. Vertical dotted lines indicate the expected beginning and end of the transit. Bottom panel: RV difference between the two transits, calculated by quadratically interpolating the 1-UT RVs on the 4-UT phases. The black line shows the fitted linear trend with a slope significant at the 4.5σ level. An offset is expected because of the different instrumental setups used in the two transits.