2010MNRAS.404.1817Z

We identify 806 ultracool dwarfs (of which 34 are newly discovered L dwarfs) from their Sloan Digital Sky Survey (SDSS) riz photometry and obtain proper motions through cross-matching with the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS) and Two-Micron All-Sky Survey (2MASS). Proper-motion and distance constraints show that nine of our ultracool dwarfs are members of widely separated binary systems: SDSS0101 (K5V+M9.5V), SDSS0207 (M1.5V+L3V), SDSS0832 (K3III+L3.5V), SDSS0858 (M4V+L0V), SDSS0953 (M4V+M9.5V), SDSS0956 (M2V+M9V), SDSS1304 (M4.5V+L0V), SDSS1631 (M5.5V+M8V) and SDSS1638 (M4V+L0V). One of these (SDSS0832) is shown to be a companion to the bright K3 giant ηCancri. Such primaries can provide age and metallicity constraints for any companion objects, yielding excellent benchmark objects. ηCancri AB is the first wide ultracool dwarf + giant binary system identified. We present new observations and analysis that constrain the metallicity of ηCancri A to be near-solar, and use recent evolutionary models to constrain the age of the giant to be 2.2-6.1Gyr. If ηCancri B is a single object, we estimate its physical attributes to be mass = 63-82M_Jup, T_eff= 1800±150K, logg = 5.3-5.5, [M/H] = 0.0 ± 0.1. Its colours are non-typical when compared with other ultracool dwarfs, and we also assess the possibility that ηCancri B is itself an unresolved binary, showing that the combined light of an L4 + T4 system could provide a reasonable explanation for its colours.