Improved parameters for the transiting hot Jupiters WASP-4b and WASP-5b

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 data with previously published transit photometry and radial velocities are R-p = 1.30(-0.04)(+0.05) R-J and M-p = 1.21-(+0.13)(0.08) M-J, resulting in a density rho(p) = 0.55(-0.02)(+0.04) rho(J). The radius and mass for the host star are R-* = 0.87 = 0.87(-0.03)(+0.04) R-circle dot and M-* = 0.85(-0.07)(+0.11) M-circle dot. Our ground-based photometry reaches 550 ppm at time sampling of similar to 50 s. 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 +/- 0.07 R-J, M-p = 1.58(-0.10)(+0.13) M-J, rho(p) = 1.23(-0.16)(+0.26) R-circle dot, and M-* = 0.96(-0.09)(+0.13) M-circle dot. The measured size of WASP-5b agrees well with the basic models of irradiated planets, while WASP-4b is clearly an anomalously large planet.