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 \aas@@fstack { \prime \prime } 1 brown dwarf binary GJ569Bab .
We took 50000 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 \pm 0.03 .
The AO+LI observations at WHT were able to resolve the binary system GJ569Bab located at 4 \aas@@fstack { \prime \prime } 92 \pm 0 \aas@@fstack { \prime \prime } 05 from GJ569A .
We measure a separation of 98.4 \pm 1.1 mas and I -band magnitudes of 13.86 \pm 0.03 and 14.48 \pm 0.03 and I - J colors of 2.72 \pm 0.08 and 2.83 \pm 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 \sim 17 at distances larger than 1″ .
The I - J colors 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 % ) more massive than Bb .
We obtain new orbital parameters which are in good agreement with those in the literature .