  Binaries are typically excluded from direct imaging exoplanet surveys.
However, the recent findings of Kepler and radial velocity programs show that
planets can and do form in binary systems. Here, we suggest that visual
binaries offer unique advantages for direct imaging. We show that Binary
Differential Imaging (BDI), whereby two stars are imaged simultaneously at the
same wavelength within the isoplanatic patch at high Strehl ratio, offers
improved point spread function (PSF) subtraction that can result in increased
sensitivity to planets close to each star. We demonstrate this by observing a
young visual binary separated by 4\asec ~with MagAO/Clio-2 at 3.9 \microns,
where the Strehl ratio is high, the isoplanatic patch is large, and giant
planets are bright. Comparing BDI to angular differential imaging (ADI), we
find that BDI's 5$\sigma$ contrast is \about 0.5 mags better than ADI's within
\about 1\asec ~for the particular binary we observed. Because planets typically
reside close to their host stars, BDI is a promising technique for discovering
exoplanets in stellar systems that are often ignored. BDI is also 2-4$\times$
more efficient than ADI and classical reference PSF subtraction, since planets
can be detected around both the target and PSF reference simultaneously. We are
currently exploiting this technique in a new MagAO survey for giant planets in
140 young nearby visual binaries. BDI on a space-based telescope would not be
limited by isoplanatism effects and would therefore be an even more powerful
tool for imaging and discovering planets.
