  The detection of trends or gradients in the transmission spectrum of
extrasolar planets is possible with observations at very low spectral
resolution. Transit measurements of sufficient accuracy using selected
broad-band filters allow for an initial characterization of the atmosphere of
the planet. We obtained time series photometry of 20 transit events and
analyzed them homogeneously, along with eight light curves obtained from the
literature. In total, the light curves span a range from 0.35 to 1.25 microns.
During two observing seasons over four months each, we monitored the host star
to constrain the potential influence of starspots on the derived transit
parameters. We rule out the presence of a Rayleigh slope extending over the
entire optical wavelength range, a flat spectrum is favored for HAT-P-12b with
respect to a cloud-free atmosphere model spectrum. A potential cause of such
gray absorption is the presence of a cloud layer at the probed latitudes.
Furthermore, in this work we refine the transit parameters, the ephemeris and
perform a TTV analysis in which we found no indication for an unseen companion.
The host star showed a mild non-periodic variability of up to 1%. However, no
stellar rotation period could be detected to high confidence.
