Discovery of a very low mass binary with the Hubble Space Telescope near-infrared camera and multiobject spectrometer

Hubble Space Telescope Near-Infrared Camera and Multiobject Spectrometer (N ICMOS) observations of six brown dwarf candidates in the Pleiades open clus ter are presented. One of them, namely CFHT-Pl-18, is clearly resolved as a binary with an angular separation of 0".33. The very low density of contam inating background stars in our images and the photometry of the components support that this system is a physical binary rather than a chance project ion. All of the available photometric and spectroscopic data indicate that the CFHT-Pl-18 system is likely a member of the Pleiades cluster, but a fin al confirmation will have to wait until lithium can be detected. Assuming c luster membership, we compare our NICMOS photometry with evolutionary model s and find that the inclusion of the effects of dust grains is necessary fo r fitting the data. We estimate that the masses of the components are about 0.045 and 0.035 M.. The binary system has a projected separation of 42 AU (for a distance of 125 pc) that is common among stellar binaries.