o Cet H₂O-maser light curve in 1995-2011. We plot the total fluxes S_tot vs. optical phase φ_s. For better visualisation, the data are repeated for a second period and binned in phase intervals with a width of 0.1 (red step function). φ_s=0 is defined as the time of maximum optical brightness. The data points marked by an asterisk are from Effelsberg, and the plusses are from Medicina data. Upper limits are shown as downward arrows in red. The sine curve (blue) was obtained by a fit to the 1995-2011 radio measurements with a period P_opt = 332 days, and it is delayed by ϕ1_ag = 0.33, that is, by 110 days.