Broad-band transmission spectrum of HAT-P-12b. The measured values are given in black. Overplotted are a cloud-free solar metallicity spectrum of HAT-P-12b from Fortney et al. (2010) (red solid line), a Rayleigh-scattering slope dr_p/dlnλ = −4H (blue solid line), and a flat line presenting wavelength-independent atmospheric absorption (green solid line). When folded with the filter, transmission curves, and a theoretical spectrum of HAT-P-12, the model values are given in their corresponding color as open circles. At the bottom the transmission curves of the used filters are shown.