Optical transmission spectrum of WASP-103b. Measurements obtained with 10 and 20 nm wide bins are shown as brown filled circles and black squares, respectively. Measurements in 2 nm wide bins centered on the Na feature, as well as on both components of the K feature are shown in magenta. Observations by Southworth & Evans (2016) are indicated by blue squares, together with their inferred spectral sloped (blue dashed line). The best-fit spectral slope inferred from our data is shown as a gray dashed line. Model transmision spectra for WASP-103b’s are shown as a light green (0.1 × solar metallicity), dark green (solar metallicity) and olive (10 × solar metallicity) lines and filled circles indicate the models binned to the resolution of the data. Below, the two dotted lines indicate the possible added spectral slopes (at 3-σ level) due to uncertainties in the properties of WASP-103b and the contaminating star. The vertical scale is identical to that of the upper panel. In the right panel, the shaded region indicates the wavelength coverage of HST/WFC3’s G141 grism, and the expected precision of spectrophotometric WFC3 measurements is indicated.