U Her H₂O maser light curve. Plotted are integrated flux densities S(tot) in the velocity range −24 < V_los < −6 km s⁻¹ in Jy km s⁻¹ vs. optical phase φ_s. For better visualization the data are repeated for a second period. φ_s = 0 is defined as the time of maximum optical brightness. Datapoints marked by an asterisk (*) are from Eſſelsberg, the plusses (+) are Medicina data. Overplotted are average integrated flux densities in phase bins of 0.1 (red), and a sine curve (blue) which was obtained by a fit to the 1990–2011 radio measurements with a period of P_opt = 405 days. The sine curve is delayed by ϕ_lag = 0.16, i.e. by 64 days with respect to the optical maximum.