| Abstract
| - Context. Ground-based simultaneous multiband transit observations allow an accurate system parameter determination and may lead to the detection and characterization of additional bodies via the transit timing variations (TTVs) method. Aims. We aim to (i) characterize the heavily bloated WASP-4b hot Jupiter and its star by measuring system parameters and the dependence of the planetary radius as a function of four (Sloan g′, r′, i′, z′) wavelengths and (ii) search for TTVs. Methods. We recorded 987 images during three complete transits with the GROND instrument, mounted on the MPG/ESO-2.2 m telescope at La Silla Observatory. Assuming a quadratic law for the stellar limb-darkening, we derived the system parameters by simultaneous fitting a composite transit light curve over all bandpasses. To compute uncertainties of the fitted parameters, we employed the bootstrap Monte Carlo method. Results. The three central transit times are measured with precision down to 6 s. We find a planetary radius Rp = 1.413 ± 0.020 RJup, an orbital inclination \hbox{$i = 88\fdg 57 \pm 0.45^{\circ}$}i=88.°57±0.45◦ and calculate a new ephemeris, a period P = 1.33823144 ± 0.00000032 days and a reference transit epoch T0 = 2 454 697.798311 ± 0.000046 (BJD). Analysis of the new transit mid-times in combination with previous measurements shows no sign of a TTV signal greater than 20 s. We perform simplified numerical simulations to place upper-mass limits of a hypothetical perturber in the WASP-4b system.
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