Disc evolution for a viscous disc with internal photoevaporation due to X-rays, using the nominal photoevaporative mass loss rates from Table 1. The host star masses vary from 0.5 M_⊙ (on the left) to 0.1 M_⊙ (on the right). Top: gas surface density as a function of disc radius, time evolution is shown in colour - from black, which corresponds to 0 Myr, to dark red, which corresponds to 10 Myr. Bottom : gaseous C/O ratio as a function of disc radius and time (colour-coded). The evaporation lines for the different molecules are given as dashed grey lines. Note that the C/O ratio is calculated from number densities and that, by definition, we have no specified C/O ratio in the gas phase beyond the CO evaporation front. We use our standard parameters for this simulation, as given in Table 3.