Upper panel: the optical/UV (black filled squares) and X-ray (red filled squares) time delays fitted with two parabolas having the same linear and quadratic terms. The solid and dashed parabolas correspond to the X-ray and optical/UV best-fit curves. Lower panel: residuals in units of σ with respect to the best-fit parabola describing the X-ray ephemeris shown in Eq. (5). (This figure is available in color in the electronic form.)