2015A&A...578A...1H

T and Y dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Up to now, there are more than 350 T dwarfs that have been identified thanks to large imaging surveys in the infrared, and their multiplicity properties can shed light on the formation process. The aim of this work is to look for companions around a sample of seven ultracoool objects. Most of them have been discovered by the WISE observatory and have not been studied before for multiplicity. We observed a sample six T dwarfs and one L9 dwarf with the Laser Guide Star (LGS) and NAOS-CONICA, the adaptive optics (AO) facility, and the near infrared camera at the ESO Very Large Telescope. We observed all the objects in one or more near-IR filters (JHK_s). From the seven observed objects, we have identified a subarcsecond binary system, WISE J0612-3036, composed of two similar components with spectral types of T6. We measure a separation of ρ=350±5mas and a position angle of PA=235±1°. Using the mean absolute magnitudes of T6 dwarfs in the 2MASS JHK_s bands, we estimate a distance of d=31±6pc and derive a projected separation of ρ∼11±2au. Another target, WISE J2255-3118, shows a very faint object at 1.3" in the K_s image. The object is marginally detected in H, and we derive a near infrared color of H-K_s>0.1mag. HST/WFC3 public archival data reveals that the companion candidate is an extended source. Together with the derived color, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-σ sensitivity limits between H=19-21 for companions at separations ≥0.5". WISE0612-3036 is probably a new T-dwarf binary composed of two T6 dwarfs. As in the case of other late T-dwarf binaries, it shows a mass ratio close to 1, although its projected separation, ∼11au, is larger than the average (∼5au). Additional observations are needed to confirm that the system is bound.