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

¹ Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland e-mail: michael.gillon@obs.unige.ch
² Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
³ School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK

Received: 7 September 2008
Accepted: 9 December 2008

Abstract

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_J and  M_J, resulting in a density  . The radius and mass for the host star are   and  . Our ground-based photometry reaches 550 ppm at time sampling of ~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  ± 0.07 R_J,  M_J,  ,  , and  . The measured size of WASP-5b agrees well with the basic models of irradiated planets, while WASP-4b is clearly an “anomalously” large planet.