  This note describes an effort to detect additional stellar sources in known
transiting exoplanet (TEP) systems, which are unresolved or barely resolved in
the Gaia Data Release 2 (DR2) catalogue. The presence of multiple unresolved
stars in photometric and spectroscopic observations of a transiting planetary
system biases measurements of the planet's radius, mass, and atmospheric
conditions. In addition to the effect on individual planetary systems, the
presence of unresolved stars across the sample of known exoplanets biases our
overall understanding of planetary systems, due to the systematic
underestimation of both masses and radii. This work uses the Astrometric
Goodness of Fit in the Along-Scan direction (GOF_AL) and the Astrometric Excess
Noise as indicators of poorly-resolved binaries. Many known close binaries in
the exoplanet host star sample have highly significant GOF_AL and Astrometric
Excess Noise values, such as WASP-20AB with Astrometric Excess Noise
significant at $4720\sigma$ and GOF_AL=124.
