  Context. The launch of the exoplanet space missions obtaining exquisite
photometry from space has resulted in the discovery of thousands of planetary
systems with very different physical properties and architectures. Among them,
the exoplanet CoRoT-29b was identified in the light curves the mission obtained
in summer 2011, and presented an asymmetric transit light curve, which was
tentatively explained via the effects of gravity darkening. Aims. Transits of
CoRoT-29b are measured with precision photometry, to characterize the reported
asymmetry in their transit shape. Methods. Using the OSIRIS spectrograph at the
10-m GTC telescope, we perform spectro-photometric differential observations,
which allow us to both calculate a high-accuracy photometric light curve, and a
study of the color-dependence of the transit. Results. After careful data
analysis, we find that the previously reported asymmetry is not present in
either of two transits, observed in July 2014 and July 2015 with high
photometric precisions of 300ppm over 5 minutes. Due to the relative faintness
of the star, we do not reach the precision necessary to perform transmission
spectroscopy of its atmosphere, but we see no signs of color-dependency of the
transit depth or duration. Conclusions. We conclude that the previously
reported asymmetry may have been a time-dependent phenomenon, which did not
occur in more recent epochs. Alternatively, instrumental effects in the
discovery data may need to be reconsidered.
