In this thesis we present an intensive analysis of public transit and radial velocity data. We used the scientific code pyaneti in order to perform a multi-planet transit and radial velocity analysis in order to derive planetary masses and radii. Results: We derived the masses, radii and compositions of the exoplanets transiting the stars K2-106, K2-141 and HD 3167. Each of these stars hosts an Ultra Short Period (USP) planet accompanied with at least one outer planetary companion. Conclusions: This thesis contributes to exoplanet community and increases our knowledge of the planetary parameters of the systems mentioned. Our characterisation represent an improvement in comparison with the previously reported in the scientific literature. This results will contribute to future developments in the study of USP planets.