Example proper motion diagrams (PMD) for separation and position angle change (left to right) from both absolute astrometric measurements (top) and from relative astrometric measurements (bottom) in the WX Cha AB system. The long dashed lines enclose the area for constant separation, as expected for a co-moving object. The dash-dotted line is the change expected if the WX Cha companion is a non-moving background star. The opening cone enclosed by the continuous lines its estimated errors. The waves of this cone show the differential parallactic motion that has to be taken into account if the other component is a non-moving background star with negligible parallax. The opening short-dashed cone represents a combination of co-motion and the maximum possible orbital motion for a circular edge-on orbit (for separation) or circular pole-on orbit (for PA). See text for more information.
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