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Context: Transmission spectroscopy has proven to be a useful tool for thestudy of exoplanet atmospheres, and has lead to the detection of a small numberof elements and molecules (Na, K, H$_2$O), but also revealed that many planetsshow flat transmission spectra consistent with the presence of opaquehigh-altitude hazes or clouds.  Aims: We apply this technique to the $M_P=0.38 M_{jup}$, $R_p=1.12 R_{jup}$,$P=2.78d$ planet WASP-49b, aiming to characterize its transmission spectrumbetween 0.73 and 1 $\mathrm{\mu}$m and search for the features of K and H$_2$O.  Methods: Three transits of WASP-49b have been observed with the FORS2instrument installed at the VLT/UT1 telescope at the ESO Paranal site. We usedFORS2 in MXU mode with grism GRIS_600z, producing simultaneous multiwavelengthtransit lightcurves throughout the i' and z' bands. We combined these data withindependent broadband photometry from the Euler and TRAPPIST telescopes toobtain a good measurement of the transit shape. Strong correlated noisestructures are present in the FORS2 lightcurves, which are due to rotatingflat-field structures that are introduced by inhomogeneities of the linearatmospheric dispersion corrector's transparency. We accounted for thesestructures by constructing common noise models from the residuals oflightcurves bearing the same noise structures, and used them together withsimple parametric models to infer the transmission spectrum.  Results: We present three independent transmission spectra of WASP-49bbetween 0.73 and 1.02 $\mu m$, as well as a transmission spectrum between 0.65and 1.02 $\mu m$ from the combined analysis of FORS2 and broadband data. Theresults obtained from the three individual epochs agree well. The transmissionspectrum of WASP-49b is best fit by atmospheric models containing a cloud deckat pressure levels of 1 mbar or lower.

FORS2 observes a multi-epoch transmission spectrum of the hot Saturn-mass exoplanet WASP-49b