  We present an algorithm that can detect blends of bright stars with fainter,
un-associated eclipsing binaries. Such systems contaminate searches for
transiting planets, in particular in crowded fields where blends are common.
Spectroscopic follow-up observations on large aperture telescopes have been
used to reject these blends, but the results are not always conclusive. Our
approach exploits the fact that a blend with a eclipsing binary changes its
shape during eclipse. We analyze original imaging data from the Optical
Gravitational Lensing Experiment (OGLE), which were used to discover planet
transit candidates. Adopting a technique developed in weak gravitational
lensing to carefully correct for the point spread function which varies both
with time and across the field, we demonstrate that ellipticities can be
measured with great accuracy using an ensemble of images. Applied to OGLE-TR-3
and OGLE-TR-56, two of the planetary transit candidates, we show that both
systems are blended with fainter stars, as are most other stars in the OGLE
fields. Moreover, while we do not detect shape change when TR-56 undergoes
transits, TR-3 exhibits a significant shape change during eclipses. We
therefore conclude that TR-3 is indeed a blend with an eclipsing binary, as has
been suggested from other lines of evidence. The probability that its shape
change is caused by residual systematics is found to be less than 0.6%. Our
technique incurs no follow-up cost and requires little human interaction. As
such it could become part of the data pipeline for any planetary transit search
to minimize contamination by blends. We briefly discuss its relevance for the
Kepler mission and for binary star detection.
