Time span of the astrometric observations dt vs. apparent separation for systems with multiple epochs and not analysed using the code Orbit. Filled circles are systems for which the Monte Carlo analysis provided some constraints on the orbits; open diamonds are systems for which no useful result could be obtained. The dashed (dot-dashed) line marks a 0.03 (0.10) coverage of the orbit assuming that the semi-major axis is equal to the apparent separation and that the system mass is 1.5 M_⊙.