Top: HIPPARCOS photometry for ε Cyg, taken around the same time as the last RVs of the McMillan data set. The displayed photometry data are median values for all cases of multiple measurements within one day; the RVs are the residuals of the McMillan measurements after subtracting the long-period signal induced by the binary companion (see Sect. 4.1), and the dashed line denotes the best-fit model. Bottom: GLS periodogram of the HIPPARCOS photometry data and of the window function of the observations. The best-fit Keplerian orbital period of the signal of 291 d is marked with a red line.