ULAS J141623.94$+$134836.3 - a faint common proper motion companion of a nearby L dwarf. Serendipitous discovery of a cool brown dwarf in UKIDSS DR6

New near-infrared large-area sky surveys (e.g. UKIDSS, CFBDS, WISE) go deeper than 2MASS and aim at detecting brown dwarfs lurking in the Solar neighbourhood which are even fainter than the latest known T-type objects, so-called Y dwarfs. Using UKIDSS data, we have found a faint brown dwarf candidate with very red optical-to-near-infrared but extremely blue near-infrared colours next to the recently discovered nearby L dwarf SDSS J141624.08$+$134826.7. We check if the two objects are co-moving by studying their parallactic and proper motion and compare the new object with known T dwarfs. The astrometric measurements are consistent with a physical pair ($sep$$\approx$75 AU) at a distance $d$$\approx$8 pc. The extreme colour ($J$$-$$K$$\approx$$-$1.7) and absolute magnitude ($M_J$=17.78$\pm$0.46 and $M_K$=19.45$\pm$0.52) make the new object appear as one of the coolest (T${}_{eff}$$\approx$600 K) and nearest brown dwarfs, probably of late-T spectral type and possibly with a high surface gravity (log $g$$\approx$5.0).