and a circular orbit, one would expect $\sim 1372$ years orbital period and, hence, up to a few mas/yr orbital motion (e.g. up to 6.7 mas/yr for an eccentricity e=0.5 for edge-on inclination), and less than $0.3 ^{\circ}$ PA change for e=0 ($\le 0.82 ^{\circ}$ PA for e=0.5 for pole-on orbit). The possible changes in separation and PA seen in Figs. 1 & 2 are smaller than these maxima, hence within the expectation.

Orbital motion is detectable as deviation among the separation and/or PA values. The data points from Feb and May 2006 in the lower left panel (Fig. 2b) are deviant from the May 2005 value (drawn as the full line for constant separation) by 4.2 & 2.6 $\sigma$, respectively, so that we have a formal significance of 4.2 & $2.6~\sigma$ for detection of orbital motion. We observe a possible small (constant, linear) change in separation, namely a decrease by $\sim 2$ to 3 mas/yr (best fit to the last six data points, see Fig. 2), in particular a better fit compared to constant separation (732.1 mas being the mean) regarding rms scatter, which has a reduced $\chi^{2} = 0.320$, and also a better fit compared to different parallaxes 