2009A&A...496..259G

The gaseous giant planets WASP-4b and WASP-5b are transiting 12-magnitude solar-type stars in the Southern hemisphere. The aim of the present work is to refine the parameters of these systems using high cadence VLT/FORS2 z-band transit photometry and high-resolution VLT/UVES spectroscopy. For WASP-4, the new estimates for the planet radius and mass from a combined analysis of our VLT data with previously published transit photometry and radial velocities are R_p=1.30^+0.05_–0.04R_J and M_p=1.21^+0.13_–0.08M_J, resulting in a density ρ_p=0.55^+0.04_–0.02ρ_J. The radius and mass for the host star are R_*=0.87^+0.04_–0.03R_☉ and M_*=0.85^+0.11_–0.07M_☉. Our ground-based photometry reaches 550ppm at time sampling of ∼50s. Nevertheless, we also report the presence of an instrumental effect on the VLT that degraded our photometry for the WASP-5 observations. This effect could be a major problem for similar programs. Our new estimates for the parameters of the WASP-5 system are R_p=1.09±0.07R_J, M_p=1.58^+0.13_–0.10M_J, ρ_p=1.23^+0.26_–0.16ρ_J, R_*=1.03^+0.06_–0.07R_☉, and M_*=0.96^+0.13_–0.09M_☉. The measured size of WASP-5b agrees well with the basic models of irradiated planets, while WASP-4b is clearly an ``anomalously'' large planet.