  We have initiated a dedicated project to follow-up with ground-based
photometry the transiting planets discovered by CoRoT in order to refine the
orbital elements, constrain their physical parameters and search for additional
bodies in the system. From 2012 September to 2016 December we carried out 16
transit observations of six CoRoT planets (CoRoT-5b, CoRoT-8b, CoRoT-12b,
CoRoT-18b, CoRoT-20b, and CoRoT-27b) at three observatories located in Germany
and Spain. These observations took place between 5 and 9 yr after the planet's
discovery, which has allowed us to place stringent constraints on the planetary
ephemeris. In five cases we obtained light curves with a deviation of the
mid-transit time of up to ~115min from the predictions. We refined the
ephemeris in all these cases and reduced the uncertainties of the orbital
periods by factors between 1.2 and 33. In most cases our determined physical
properties for individual systems are in agreement with values reported in
previous studies. In one case, CoRoT-27b, we could not detect any transit event
in the predicted transit window.
