Velocities of the local stellar surface hidden behind the planet during its transit. The observations of the two nights are shown in dark blue and green, respectively. The first to fourth contacts are traced with the vertical dashed lines. The overlap region between the local stellar and atmospheric signals (between phases −0.025 and 0) is shown in red and is not used for the fit. The spin-orbit misalignment is modeled with a solid body rotation (in orange), and with a differential rotation (in light blue). The differential rotation model is slightly preferred.