Top : spatial distribution of members stars of NGC 6871 in Cartesian coordinates. Arrows show the velocity vectors on the plane of the sky; their lengths are proportional to the speed on the plane of the sky, while their colors map the radial component (v_RAD) of the velocity. Positive values point outward. Bottom : distribution of members in the v_RAD—R diagram. The black line represents the bet-fit value for 〈v_rad〉, and the dashed gray area its uncertainties.