  We present near-infrared high-contrast imaging photometry and integral field
spectroscopy of ROXs 42B, a binary M0 member of the 1--3 Myr-old $\rho$
Ophiuchus star-forming region, from data collected over 7 years. Each data set
reveals a faint companion -- ROXs 42Bb -- located $\sim$ 1.16" ($r_{proj}$
$\approx$ 150 $AU$) from the primaries at a position angle consistent with a
point source identified earlier by Ratzka et al. (2005). ROXs 42Bb's astrometry
is inconsistent with a background star but consistent with a bound companion,
possibly one with detected orbital motion. The most recent data set reveals a
second candidate companion at $\sim$ 0.5" of roughly equal brightness, though
preliminary analysis indicates it is a background object. ROXs 42Bb's $H$ and
$K_{s}$ band photometry is similar to dusty/cloudy young, low-mass late M/early
L dwarfs. $K$-band VLT/SINFONI spectroscopy shows ROXs 42Bb to be a cool
substellar object (M8--L0; $T_{eff}$ $\approx$ 1800--2600 $K$), not a
background dwarf star, with a spectral shape indicative of young, low surface
gravity planet-mass companions. We estimate ROXs 42Bb's mass to be 6--15
$M_{J}$, either below the deuterium burning limit and thus planet mass or
straddling the deuterium-burning limit nominally separating planet-mass
companions from other substellar objects. Given ROXs 42b's projected separation
and mass with respect to the primaries, it may represent the lowest mass
objects formed like binary stars or a class of planet-mass objects formed by
protostellar disk fragmentation/disk instability, the latter slightly blurring
the distinction between non-deuterium burning planets like HR 8799 bcde and
low-mass, deuterium-burning brown dwarfs.
