The rate of discoveries of transiting planets is dictated by the need of ground based follow-up observations, needed to verify the planetary nature of the transit candidates. Candidate detections have been obtained for about 2,3% of all COROT targets, but finding periodic transit events isn't enough to claim a planet discovery, since several configurations could mimic a transiting planet, such as stellar binaries, or an eclipsing fainter star very close to the target star, whose light, blended in the light curve, can reproduce transit-like events. A first screening is executed on the light curves, searching hints of secondary eclipses or a rather V-shaped transit, indicative of a stellar nature of the transits. For the brighter targets, the prism in front of the exoplanets CCDs provides photometry in 3 different colors, enabling to reject planet candidates that have different transit depths in the three channels, a behaviour typical of binary stars. These tests allow to discard 83% of the candidate detections, whilst the remaining 17% are screened with photometric and radial velocity follow-up from a network of telescopes around the world. Photometric observations, required to rule out a possible contamination by a diluted eclipsing binary in close vicinity of the target, is performed on several 1 m-class instruments, but also employs the 2 m Tautenburg telescope in Germany and the 3,6 m CFHT/Megacam in Hawaii. The radial velocity follow-up allows to discard binaries or even multiple star system and, given enough observations, provide the mass of the exoplanets found. Radial velocity follow-up is performed high-precision spectrographs, namely SOPHIE, HARPS and HIRES. Once the planetary nature of the candidate is established, high-resolution spectroscopy is performed on the host star, in order to accurately determine the stellar parameters, form which further derive the exoplanet characteristics. Such work is done with large aperture telescopes, as the UVES spectrograph or HIRES.
Interesting transiting planets could be further followed-up with the infrared Spitzer Space Telescope, to give an independent confirmation at a different wavelength and possibly detect reflected light from the planet or the atmospheric compositions. COROT-7b and COROT-9b have already been observed by Spitzer.
Papers presenting the results of follow-up operations of planetary candidates in the IRa01, LRc01, LRa01, SRc01 fields have been published. Sometimes the faintness of the target star or its characteristics, such as a high rotational velocity or strong stellar activity, do not allow to determine unambiguously the nature or the mass of the planetary candidate.
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