September 2009 light curves of RS Cae in time bins of 100 s, folded on the orbital period with the center of the X-ray dips defining phase zero (Eq. (1)). The grey area marks the interval of high background activity during the XMM-Newton pointing. a)–b) Energy-resolved EPIC/pn light curves. c) Corresponding hardness ratios HR_XMM = (H_X − S_X)/(H_X + S_X). d) Optical and ultraviolet light curves measured subsequently with three filters at the optical monitor. e) Optical B-band light curve with a time resolution of about 227 s, plotted twice and shifted by − 3 and − 2 orbital cycles, respectively.