Three simulated transit light curves (top panels) and stellar disc outputs (bottom panels), generated by PRISM. Light curves: the solid lines represent the noise-free synthetic light curves containing the starspot anomaly, while, the filled circles represent the spot-free synthetic light curves (see Sect. 3) with added Gaussian noise. Stellar discs: the central solid lines represents the stellar equator (assuming orbital alignment) and the upper and lower solid lines represent the upper and lower boundaries of the transit cords. Because the high ρ_spot, the starspot boundary is highlighted with a black circular boarder. The observational wavelength and rms scatter for the simulated transits are set at 1000 nm and 200 ppm, respectively. These were generated using a 1.0 R_⊕ Earth-sized planet transiting a 3200 K, 0.155 R_⊙ M4V dwarf star with i = 90.0° and P = 1 d. The starspotproperties were set at θ = 0°, ϕ = 90° and T_spot = 3150 K (ρ_spot = 0.93). Left panels: r_spot = 0.087R_*, middle panels: r_spot = 0.175R_*, right panels: r_spot = 0.262R_*.