Evidence of Orbital Motion in the Binary Brown Dwarf Kelu-1AB

We have resolved Kelu-1 into a binary system with a separation of ~290 mas using the Laser Guide Star Adaptive Optics system on the Keck II telescope. We have also reanalyzed a 1998 HST observation of Kelu-1 and find that the observed PSF is best fit by a binary object separated by 45 mas. Observations at multiple epochs confirm that the two objects share a common proper motion and clearly demonstrate the first evidence of orbital motion. Kelu-1B is fainter than Kelu-1A by 0.39+/-0.01 mag in the K^' filter and 0.50+/-0.01 mag in the H filter. We derive spectral types of L2+/-1 and L3.5+/-1 for Kelu-1A and B, respectively. The separation of flux into the two components rectifies Kelu-1's overluminosity problem, which has been known for quite some time. Given the available data, we are able to constrain the inclination of the system to >81° and the orbital period to >~40 yr.