  Clouds have been shown to be present in many exoplanetary atmospheres. Cloud
formation modeling predicts considerable inhomogeneities of cloud cover,
consistent with optical phase curve observations. However, optical phase curves
cannot resolve some existing degeneracies between cloud location and cloud
optical properties.
  We present a conceptually simple technique to detect inhomogeneous cloud
cover on exoplanets. Such an inhomogeneous cloud cover produces an asymmetric
primary transit of the planet in front of the host star. Asymmetric transits
produce characteristic residuals compared to a standard symmetric model.
Furthermore, bisector spans can be used to determine asymmetries in the transit
light curve.
  We apply a model of asymmetric transits to the light curves of HAT-P-7b,
Kepler-7b and HD209458b and search for possible cloud signatures. The nearly
uninterrupted Kepler photometry is particularly well-suited for this method
since it allows for a very high time resolution.
  We do not find any statistically sound cloud signature in the data of the
considered planets. For HAT-P-7b, a tentative detection of an asymmetric cloud
cover is found, consistent with analysis of the optical phase curve. Based on
Bayesian probability arguments, a symmetric model with an offset in the transit
ephemeris remains, however, the most viable model. Still, this work
demonstrates that for suitable targets, namely low-gravity planets around
bright stars, the method can be used to constrain cloud cover characteristics
and is thus a helpful additional tool to study exoplanetary atmospheres.
