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Title: Lucky Imaging Adaptive Optics of the brown dwarf binary GJ569Bab
Authors: Autors: B. Femenía (1 and 2), R. Rebolo (1 and 3), J. A. Pérez-Prieto (1), S. R. Hildebrandt (1 and 4), L. Labadie (1 and 2), A. Pérez-Garrido (5), V. J. S. Béjar (1 and 2), A. Díaz-Sánchez (5), I. Villó (5), A. Oscoz (1), R. López (1), L. F. Rodríguez (1), J. Piqueras (1 and 6) ((1) Intituto de Astrofísica de Canarias-SPAIN, (2) Departamento de Astrofísica Universidad de La Laguna-SPAIN, (3) CSIC-SPAIN, (4) Laboratoire de Physique Subatomique et Cosmologie-France, (5) Universidad Politécnica de Cartagena-SPAIN, (6) Max-Planck-Institut für Sonnensystemforschung-Germany)

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\farcs1 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±0.03. The AO+LI observations at WHT were able to resolve the binary system GJ569Bab located at 4\farcs 92 ±0\farcs05 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 I-J 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\sim 17 at distances larger than 1\arcsec. The I-J 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%) more massive than Bb. We obtain new orbital parameters which are in good agreement with those in the literature.

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