Presented in “Living Together: Planets, Host Stars and Binaries” workshop, September, 8 – 12, 2014, Litomysl, Czech Republic

Determining masses and radii of extrasolar planets with high precision is key to our understanding of their chemical composition, internal structure, and thereby their formation and evolution. Toward this goal we are applying the defocusing technique for the photometric observation of selected planetary systems with the 1 m Turkish telescope T100 at the TÜB˙ITAK National Observatory (TUG). In this contribution we present preliminary analyses of transit light curves obtained using this technique for the exoplanets KELT-3b, HAT-P-10b/WASP-11b, HAT-P-20b, and HAT-P-22b.