  All transiting planet observations are at risk of contamination from nearby,
unresolved stars. Blends dilute the transit signal, causing the planet to
appear smaller than it really is, or produce a false positive detection when
the target star is blended with an eclipsing binary. High spatial resolution
adaptive optics images are the best way of resolving undetected contaminants.
Here we present companions and detection limits for 12 Kepler candidates, of
which 4 have companions within 4 arcsec. One system (KOI 1537) consists of two
similar-magnitude stars separated by 0.1 arcsec, while KOI 174 has a companion
at 0.5 arcsec. In addition, observations were made of 15 transiting planets
that were previously discovered by other surveys. The only companion found
within 1 arcsec of a known planet is the previously identified companion to
WASP-2b. An additional four systems have companions between 1-4 arcsec:
HAT-P-30b (3.7 arcsec, Delta Ks = 2.9), HAT-P-32b (2.9 arcsec, Delta Ks = 3.4),
TrES-1b (2.3 arcsec, Delta Ks = 7.7), and WASP-P-33b (1.9 arcsec, Delta Ks =
5.5), some of which have not been reported previously. Depending on the spatial
resolution of the transit photometry for these systems, these companion stars
may require a reassessment of the planetary parameters derived from transit
light curves. For all systems observed, we report the limiting magnitudes of
additional objects located 0.1-4 arcsec from the target.
