2010A&A...515A..92S

Completing the poorly known substellar census of the solar neighbourhood, especially with respect to the coolest brown dwarfs, will lead to a better understanding of failed star formation processes and binary statistics with different environmental conditions. Using UKIDSS data and their cross-correlation with the SDSS, we searched for high proper motion mid- to late-T dwarf candidates with extremely blue near-infrared (J-K<0) and very red optical-to-near-infrared (z-J>+2.5) colours. With 11 newly found T dwarf candidates, the proper motions of which range between 100 and 800mas/yr, we increased the number of UKIDSS T dwarf discoveries by ≃30%. Large proper motions were also measured for six of eight previously known T4.5-T9 dwarfs detected in our survey. All new candidates can be classified as T5-T9 dwarfs based on their colours. Two of these objects were found to be common proper motion companions of Hipparcos stars with accurate parallaxes. The latter allow us to determine absolute magnitudes from which we classify Hip 63510C as T7 and Hip 73786B as T6.5 dwarfs with an uncertainty of ±1 spectral subtype. The projected physical separation from their low-mass (M0.5 and K5) primaries is in both cases about 1200AU. One of the Hipparcos stars has already a known very low-mass star or brown dwarf companion on a close astrometric orbit (Hip 63510B = Gl 494B). With distances of only 11.7 and 18.6pc, deduced from their primaries respectively for Hip 63510C and Hip 73786B, various follow-up observations can easily be carried out to study these cool brown dwarfs in more detail and to compare their properties with those of the already well-investigated primaries.