Sky-subtracted, wavelength-calibrated spectra for TrES-3 (dark blue line) and the simultaneously observed comparison star (orange line), both normalised with an arbitrary but common normalisation factor. The parts masked out during the generation of spectrum-masked light curves (see text) are shown with a lighter shade and the light orange vertical lines show the ranges used to generate the 16 narrowband light curves. The dotted vertical lines show the Ka I and Na I resonance doublets.