2mass J05185995−2828372: Discovery of an Unresolved L/t Binary

We present the peculiar near-infrared spectrum of the newly discovered brown dwarf 2MASS J05185995−2828372, identified in the Two Micron All Sky Survey. Features characteristic of both L and T dwarfs are present, namely strong carbon monoxide absorption in K-band, strong methane absorption in Jand H-bands, and red near-infrared colors. We consider several scenarios that could produce these features and conclude that the object is most likely to be an unresolved L/T binary system. We discuss how the estimated photometric properties of this object are consistent with the observed J-band brightening of brown dwarfs between late-L and early-T dwarfs, making detailed study of this system an important probe of the L/T transition. Subject headings: binaries:general—stars: low-mass, brown dwarfs—stars: individual (2MASS J05185995−2828372)