Recently , Neuhäuser et al .
( 2005a ) presented evidence for a sub-stellar , common proper motion companion to GQ Lup .
With two theoretical mass estimates , both below the Deuterium burning minimum mass limit , the companion is probably a planet imaged directly .
We present here a more detailed astrometric analysis of the GQ Lup system , using all the ( different ) proper motions published for the primary .
The common proper motion is significant in all cases , also when taking into account the error in parallax or distance ( 140 \pm 50 pc ) .
When using the weighted mean , the significance for common proper motion of GQ Lup and its companion is 7 \sigma + 4 \sigma for no change in separation plus 8 \sigma for no change in position angle .
We also discuss the question , whether GQ Lup and its common-proper motion companion are not bound , but share the same or similar proper motion as two independent members of the Lupus T association , which is a moving group , where most members should have the same motion anyway .
Given our discussion , this hypothesis can be rejected by several \sigma : The probability to find by chance an L-dwarf fainter than { K _ { S } } = 14 mag within 0.7325 ^ { \prime \prime } with ( almost ) the same proper motion of GQ Lup is only { \leq 3 \cdot 10 ^ { -10 } } .
The orbital motion of the system is not yet detected ( 1.4 \pm 2.2 mas/yr ) , but is probably smaller than the escape velocity ( 5.3 \pm 2.1 mas/yr ) , so that the system may well be gravitationally bound and stable .
This is different for the 2MASSWJ 1207334-393254 system , as we also show .