Observed light variations of RU Vir (cf. Sect. 3) in different filters as labelled on the right hand side. The data were adopted from Eggen (1975a; BVRI) and Whitelock et al. (2006; JHKL), respectively. Measurements from different periods are merged into a combined light cycle, each data point is then plotted twice to highlight the periodic variations. Sinusoidal fits (one Fourier component, see text for details) are overplotted with dotted lines to guide the eye. An analogous plot for an M-type Mira, namely RR Sco, can be found in Lattanzio & Wood (2004; their Fig. 2.44).