The color signature of the transit of HD 209458: Discrepancies between   stellar atmospheric models and observations
  Exoplanetary transits produce a double-horned color signature that is distinct from both binaries and blends and can thus be used to separate exoplanets from false positives in transit searches. Color photometry with precision sufficient to detect this signal in transits of HD 209458 is available in the literature. Analysis of these observations reveals that, while the signature does exhibit the expected shape, it is significantly stronger than PHOENIX atmospheric models predict. 
B. Tingley, C. Thurl & P. D. Sackett