The Transiting System GJ1214: High-Precision Defocused Transit   Observations and a Search for Evidence of Transit Timing Variation
  Aims: We present 11 high-precision photometric transit observations of the transiting super-Earth planet GJ1214b. Combining these data with observations from other authors, we investigate the ephemeris for possible signs of transit timing variations (TTVs) using a Bayesian approach.   Methods: The observations were obtained using telescope-defocusing techniques, and achieve a high precision with random errors in the photometry as low as 1mmag per point. To investigate the possibility of TTVs in the light curve, we calculate the overall probability of a TTV signal using Bayesian methods.   Results: The observations are used to determine the photometric parameters and the physical properties of the GJ1214 system. Our results are in good agreement with published values. Individual times of mid-transit are measured with uncertainties as low as 10s, allowing us to reduce the uncertainty in the orbital period by a factor of two.   Conclusions: A Bayesian analysis reveals that it is highly improbable that the observed transit times is explained by TTV, when compared with the simpler alternative of a linear ephemeris. 
K. B. W. Harps{\o}e, S. Hardis, T. C. Hinse, U. G. J{\o}rgensen, L.   Mancini, J. Southworth, K. A. Alsubai, V. Bozza, P. Br...
astro-ph.EP