  We have designed and built the first band-limited coronagraphic mask used for
ground-based high-contrast imaging observations. The mask resides in the focal
plane of the near-infrared camera PHARO at the Palomar Hale telescope and
receives a well-corrected beam from an extreme adaptive optics system. Its
performance on-sky with single stars is comparable to current state-of-the-art
instruments: contrast levels of $\sim10^{-5}$ or better at 0.8" in $K_s$ after
post-processing, depending on how well non-common-path errors are calibrated.
However, given the mask's linear geometry, we are able to conduct additional
unique science observations. Since the mask does not suffer from pointing
errors down its long axis, it can suppress the light from two different stars
simultaneously, such as the individual components of a spatially resolved
binary star system, and search for faint tertiary companions. In this paper, we
present the design of the mask, the science motivation for targeting binary
stars, and our preliminary results, including the detection of a candidate
M-dwarf tertiary companion orbiting the visual binary star HIP 48337, which we
are continuing to monitor with astrometry to determine its association.
