Proper motion diagram of S Ori 70, S Ori 73, and J0538−0213 (solid circles and labeled). All the reference sources within an area of 0.25 arcmin² (S Ori 70), 4.60 arcmin² (S Ori 73), and 6.25 arcmin² (J0538−0213) are plotted as small dots; those defining the astrometric transformation equations are in red. The ellipses around the S Ori targets represent their 2σ proper motion uncertainties. The Hipparcos motion of the σ Orionis cluster is depicted with a gray solid-line ellipse.