Polynomial plus sinusoidal fit (thick black line) of the observed O–C values (yellow dots, OC4 in Table 1). From darker to lighter shading, the blue regions show the 1, 2 and 3σ limits of the predicted O–C ranges, assuming errors in all estimated parameters, including P₂, the period of the long-term variation. The inset shows the deviation from the best fit to the currently available data.