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In this PhD dissertation, I discuss issues of the Radial Velocities (RV) andtransit methods. These techniques allow us to derive the mass and radius of anexoplanet, necessary to model its bulk structure and to have insight on itsformation. To do this, however, also the same parameters of its host star areneeded. By using spectroscopy, I participated in TRANSITS, an RV follow-upprogram of Kepler Objects of Interest. I determined the parameters of nine hoststars, enabling the characterization of their companions. With the same method,I participated in two studies which aim at exploring the mass-radiusrelationship of low-mass stars and at improving the statistics of star-planetinteractions. I also inspected the behavior of SOPHIE/OHP spectra forinstrumental effects which can affect the measure of the stellar parameters.From a different perspective, I studied Kepler-117, a multi-planetary systemwhich presents Transit Timing Variations (TTV). A specific approach wasdeveloped in order to realize a simultaneous fit of transits, RV, and TTV, usedto measure the mass of the lightest planet of the system, which was poorlyconstrained by RV alone. Finally, I focused on the impact of stellar activityin transit photometry and RV. This phenomenon affects the determination of theplanet radius and mass, as well as other key parameters. I implemented twostarspot modeling codes into a Markov Chain Monte Carlo software, and addedspot evolution to one of them. I applied the codes to observations of the Sun,CoRoT-2, and CoRoT-7. In particular, I carried out an extensive study on thelight curve of CoRoT-2, and explored the effects of the spots on the transitparameters. The planetary systems studied in this work provide furtherconstraints on models of planet interior and formation. With the techniqueshere developed, they help in the preparation of future exoplanet surveys.

Characterization of transiting exoplanets: analyzing the impact of the host star on the planet parameters