Astrometry derived from previous interferometric measurements (see Table 1). Positions of the Aa1 components with respect to Ac1 are plotted (blue), as are Ac2 with respect to Ac1 (green; in the very centre). Tiny oscillations visible as thickness of lines are due to photocentre motions. We used positive signs of (u, v) for this plot. The orbit is constrained not only by this astrometry, but also by ETVs and RVs (see Figs. 6 and 8).