the two stars (of this hypothetical binary) and their brightness difference. If the observed change in separation between GQ Lup A and b was due to this effect, the time-scale is short, of the order of months to few years, so the separation between the two stars would be limited to a few AU. Even for large mass and brightness differences between the two stars, the effect would then be limited to about 0.14 mas/yr. The observed effect is much larger ($5.3 \pm 1.1$ mas/yr, comparing the separations in 2005.40 and 2006.14), hence cannot be explained by this effect.\
(ii) GQ Lup A and its apparent companion b do not orbit each other, but have slightly different proper motions. This possibility has already been discussed in Mugrauer & Neuhäuser (2005). Since signatures of accretion are detected in both objects (A and companion; Seifahrt et al. 2007), both are young and, hence, are most likely members of the Lupus I could. The velocity dispersion in star forming regions like Lupus is only a few mas/yr, the same order of magnitude as the difference in proper motion observed here. The probability, however, to find two young objects within $\sim 0.7^{\prime \prime}$ with 