Apparent DM variation reproduced by de-dispersing the simulated data with a non-binary model (shown as blue points), that is, ignoring the dispersive Doppler variation (DDV). The data are plotted against pulsar’s angular orbital position ψ = ω + θ (ω is the longitude of periastron and θ is the relativistic true anomaly) with respect to the ascending node. Both amplitude and phase are consistent with Fig. 2 in Ransom et al. (2004), given that ω ~ 80°. The dashed horizontal line indicates the DM used in the simulation, and the orange line represents the calculation with Eq. (9), which matches perfectly with the simulation.