  We report the discovery of a substellar companion to the nearby solar-type
star HD 46588 (F7V, 17.9 pc, ~3 Gyr). HD 46588 B was found through a survey for
common proper motion companions to nearby stars using data from the Wide-field
Infrared Survey Explorer and the Two-Micron All-Sky Survey. It has an angular
separation of 79.2" from its primary, which corresponds to a projected physical
separation of 1420 AU. We have measured a spectral type of L9 for this object
based on near-infrared spectroscopy performed with TripleSpec at Palomar
Observatory. We estimate a mass of 0.064+0.008/-0.019 Msun from a comparison of
its luminosity to the values predicted by theoretical evolutionary models for
the age of the primary. Because of its companionship to a well-studied star, HD
46588 B is one of the few known brown dwarfs at the L/T transition for which
both age and distance estimates are available. Thus, it offers new constraints
on the properties of brown dwarfs during this brief evolutionary phase. The
discovery of HD 46588 B also illustrates the value of the Wide-field Infrared
Survey Explorer for identifying brown dwarfs in the solar neighborhood via
their proper motions.
