Two-colour diagrams with a grid of “star + spot” models, shown with the bow-shaped curves. The numbers along the curves represent the amount of veiling in each model. In the left panel, the cluster of dots are the observed colours of DR Tau. In the right panel, a correction for interstellar reddening A_V = 1ṃ0 has been applied. The slope of the interstellar reddening is indicated with an arrow.