Upper plot: O - C_lin versus time in years. O is the moment of the zero phase and C_lin = M₀₁ + P₁ × E. M₀₁ and P₁ are the parameters of the linear approximation of O(E) behavior, E is the number of rotational cycles elapsed from the initial epoch M₀₁ and P₁ is the mean rotational period in the interval 1970–2018; M₀₁ = 2 452 650.9048 and P₁ = 0.52070819 d. Nonlinearityof O - C_lin is a result of variable rotational period P. O - C_lin is fitted by a fourth-order polynomial (solid line). Bottom plot: residuals O - C_pol4 show that the model ensures the accuracy of ephemeris 0.0025 d or 0.005 in phase. Optical photometry is denoted using yellow circles and UV spectrophotometry derived from observed fluxes using green squares. Sizes of the symbols are inversely proportional to the square root of the uncertainties of the determinations of O times derived from individual data sources.