• instrumentation: adaptive optics;
  • instrumentation: high angular resolution;
  • binaries: close;
  • brown dwarfs;
  • stars: individual: GJ569;
  • stars: low-mass


The potential of combining Adaptive Optics (AO) and Lucky Imaging (LI) to achieve high-precision astrometry and differential photometry in the optical is investigated by conducting observations of the close 0.1 arcsec brown dwarf binary GJ569Bab. We took 50 000 I-band images with our LI instrument FastCam attached to NAOMI, the 4.2-m William Herschel Telescope (WHT) AO facility. In order to extract the most of the astrometry and photometry of the GJ569Bab system we have resorted to a PSF fitting technique using the primary star GJ569A as a suitable PSF reference which exhibits an I-band magnitude of 7.78 ± 0.03. The AO+LI observations at WHT were able to resolve the binary system GJ569Bab located at 4.92 ± 0.05 arcsec from GJ569A. We measure a separation of 98.4 ± 1.1 mas and I-band magnitudes of 13.86 ± 0.03 and 14.48 ± 0.03 and IJ colours of 2.72 ± 0.08 and 2.83 ± 0.08 for the Ba and Bb components, respectively. Our study rules out the presence of any other companion to GJ569A down to magnitude I∼ 17 at distances larger than 1 arcsec. The IJ colours measured are consistent with M8.5-M9 spectral types for the Ba and Bb components. The available dynamical, photometric and spectroscopic data are consistent with a binary system with Ba being slightly (10–20 per cent) more massive than Bb. We obtain new orbital parameters which are in good agreement with those in the literature.