Keck imaging of binary L dwarfs

We present Keck near-infrared imaging of three binary L dwarf systems, all of which are likely to be substellar. Two are Lithium dwarfs, and a third e xhibits an L7 spectral type, making it the coolest binary known to date. Al l have component flux ratios near 1 and projected physical separations betw een 5 and 10 AU, assuming distances of 18-26 pc from recent measurements of trigonometric parallax, These surprisingly similar binaries represent the sole detections of companions in 10 L dwarf systems that were analyzed in t he preliminary phase of a much larger dual-epoch imaging survey. The detect ion rate prompts us to speculate that binary companions to L dwarfs are com mon, that similar-mass systems predominate, and that their distribution pea ks at radial distances in accord both with M dwarf binaries and with the ra dial location of Jovian planets in our own solar system. To fully establish these conjectures against doubts raised by biases inherent in this small p reliminary survey, however, will require quantitative analysis of a larger volume-limited sample that has been observed with high resolution and dynam ic range.