the most likely cause for the small (linear) variation in the separation; quantitatively, the variation is within expectation.

The parallax of GQ Lup
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The NACO observations were also used to determine the parallax of GQ Lup A and its companion. The largest error source in the luminosity of the companion and, hence, its mass determination is the uncertain distance (90 to 190 pc for Lupus I, N05). In the north-west corner of the small (S13) NACO field-of-view (FoV), there is an additional star detected in all NACO images, about 6.4$^{\prime \prime}$ NW of GQ Lup A. As any faint object near a bright star, we have to see it as a companion candidate, which we call GQ Lup/cc2 ([*cc*]{} for [*c*]{}ompanion [*c*]{}andidate, GQ Lup b formerly was GQ Lup/cc1). See Table 3 for the separation of this object (relative to both GQ Lup A and its confirmed companion b) corrected for pixel scale and detector orientation; the largest contribution to the error budget is due to the large separation in pixels (several hundred pixels) between the object cc2 and the two components of the GQ Lup system, to be multiplied by the error in the pixel scale, and then 