Structure-optimised DM of R1 at 1400 MHz as measured at different epochs. The two Arecibo data points are from Scholz et al. (2016) and Hessels et al. (2019), respectively. For Apertif, we averaged the DM of the four bursts for which we determined a structure-optimised DM. The error bars indicate 1σ uncertainties. The uncertainty on the second data point is smaller than the circle. The DM has significantly increased from 560.57(7) pc cm⁻³ in September 2016 to 563.5(2) in November 2018.