Radio timing observations obtained with the Lovell, Nançay, Parkes, and Westerbork telescopes. (a): timing observations as a function of observing frequency. (b) and (c): the residuals of the timing model as a function of orbital phase and time from observations with Lovell, Nançay, and Parkes. (d): the residuals of the timing model as a function of orbital phase from observations with Westerbork. The orbital phases covered by the WSRT observations is indicated with square brackets. Orbital phase ϕ = 0 is defined at the time of ascending node.