We have used blue near-infrared colours to select a group of 12 spectroscopically-confirmed UKIDSS T dwarfs later than T4 .
From amongst these we identify the first two kinematic halo T-dwarf candidates .
Blue near-infrared colours have been attributed to collisionally-induced hydrogen absorption , which is enhanced by either high surface gravity or low metallicity .
Proper motions are measured and distances estimated , allowing the determination of tangential velocities . U and V components are estimated for our objects by assuming V _ { rad } = 0 .
From this , ULAS J0926+0835 is found to have U = 62 km s ^ { -1 } and V = -140 km s ^ { -1 } and ULAS J1319+1209 is found to have U = 192 km s ^ { -1 } and V = -92 km s ^ { -1 } .
These values are consistent with potential halo membership .
However , these are not the bluest objects in our selection .
The bluest is ULAS J1233+1219 , with J - K = -1.16 \pm 0.07 , and surprisingly this object is found to have young disc-like U and V .
Our sample also contains Hip 73786B , companion to the metal-poor K5 dwarf Hip 73786 .
Hip 73786 is a metal-poor star , with [ Fe / H ] = -0.3 \pm 0.1 and is located at a distance of 19 \pm 0.7 pc . U,V,W space velocity components are calculated for Hip 73786A and B , finding that U = -48 \pm 7 km s ^ { -1 } , V = -75 \pm 4 km s ^ { -1 } and W = -44 \pm 8 km s ^ { -1 } .
From the properties of the primary , Hip 73786B is found to be at least 1.6 Gyr old .
As a metal poor object , Hip 73786B represents an important addition to the sample of known T dwarf benchmarks .