Proper motion diagrams for S Ori 70 (left panel, Ω2000/HAWK-I data) and S Ori 73 (right panel, ISAAC/HAWK-I data). The two objects of interest are labelled. All identified sources within an area of 4 arcmin² around the targets are plotted as small dots. The sources used as astrometric references are shown with red tiny dots. The ellipses around S Ori 70 and 73 represent the 2-σ proper motion uncertainties. The Hipparcos motion of the σ Orionis cluster is depicted with a solid-line ellipse.