---
author:
- Ralph Neuhäuser
- Markus Mugrauer
- Andreas Seifahrt
- 'Tobias O.B. Schmidt'
- Nikolaus Vogt
date: 'Received 16 Aug 2007; accepted 4 Dec 2007 '
title: 'Astrometric and photometric monitoring of GQ Lup and its sub-stellar companion[^1]'
---

[ Neuhäuser et al. (2005) presented direct imaging evidence for a sub-stellar companion to the young T Tauri star GQ Lup. Common proper motion was highly significant, but no orbital motion was detected. Faint luminosity, low gravity, and a late-M/early-L spectral type indicated that the companion is either a planet or a brown dwarf. ]{} [ We have monitored GQ Lup and its companion in order to detect orbital and parallactic motion and variability in its brightness. We also search for closer and fainter companions. ]{} [ We have taken six more images with the VLT Adaptive Optics instrument NACO from May 2005 to Feb 2007, always with the same calibration binary from Hipparcos for both astrometric and photometric calibration. By adding up all the images taken so far, we search for additional companions. ]{} [ The position of GQ Lup A and its companion compared to a nearby non-moving background object varies as expected for parallactic motion by 