  The formation of massive planetary or brown dwarf companions at large
projected separations from their host star is not yet well understood. In order
to put constraints on formation scenarios we search for signatures in the orbit
dynamics of the systems. We are specifically interested in the eccentricities
and inclinations since those parameters might tell us about the dynamic history
of the systems and where to look for additional low-mass sub-stellar
companions. For this purpose we utilized VLT/NACO to take several well
calibrated high resolution images of 6 target systems and analyze them together
with available literature data points of those systems as well as Hubble Space
Telescope archival data. We used a statistical Least-Squares Monte-Carlo
approach to constrain the orbit elements of all systems that showed significant
differential motion of the primary star and companion. We show for the first
time that the GQ Lup system shows significant change in both separation and
position angle. Our analysis yields best fitting orbits for this system, which
are eccentric (e between 0.21 and 0.69), but can not rule out circular orbits
at high inclinations. Given our astrometry we discuss formation scenarios of
the GQ Lup system. In addition, we detected an even fainter new companion
candidate to GQ Lup, which is most likely a background object. We also updated
the orbit constraints of the PZ Tel system, confirming that the companion is on
a highly eccentric orbit with e > 0.62. Finally we show with a high
significance, that there is no orbital motion observed in the cases of the DH
Tau, HD 203030 and 1RXS J160929.1-210524 systems and give the most precise
relative astrometric measurement of the UScoCTIO 108 system to date.
