Colour − colour diagram containing observations at several epochs for a sample of C-type Miras adopted from Whitelock et al. (2006) as listed in their Table 2 (no correction for interstellar reddening). In addition, the dashed line marks the average variation of RU Vir during the light cycle as derived from the sinusoidal fits in Fig. 8. The arrow marks the direction of how the star passes through the loop.