AbstractTransiting extrasolar planets provide unmatched insights into the structure and composition of close-in planets. When a planet transits its host star, its radius is known, which together with radial velocity measurements, allows accessing the planetary density. We present results obtained using the Euler and TRAPPIST telescopes that aim at reaching very high accuracy on the parameters derived from transit lightcurves. Here, we show the case of the recently discovered WASP-42b and WASP-49b and new observations of WASP-50b.