XO-2b narrowband GTC photometric results at four wavelengths (colored points) along with the z-band radius (black dot) of Fernandez et al. (2009). The X-axis error bars indicate the wavelength range of the photometry, while the Y-axis error bars indicates the errors on the measured planetary radius, assuming a stellar radius of 0.976 R_⊙. Also over-plotted is a 1500 K isothermal model (binned to R = 500) of the transmission spectrum from Fortney et al. (2010) adapted for XO-2b, which is dominated by gaseous Na and K. TiO and VO opacities are removed in this model. The open circles indicate the predicted filter-integrated model values.