Simulated R_p/R_star for five transits observed with the future ESPRESSO spectrograph on the VLT (blue squares). Observations of five transits by the STIS spectrograph on HST as presented by Pont et al. (2013) are indicated by red circles. These are shifted slightly redward for clarity. All error bars indicate 1σ uncertainty intervals. These are larger by between 1.5 and 2 times for the ESPRESSO than for the STIS observations.