Top: histogram of normalised DM offsets, ΔDM = DM_ref − DM_HBA. Values of ΔDM are normalised by the quadrature sum of DM offset uncertainty, σ_ΔDM, and δDM_orb (see text). Bottom: dependence of normalised ΔDM on the epoch offset between reference catalogue epoch and LOFAR epoch of DM measurements. Isolated MSPs are shown with open circles, and binary MSPs with filled circles. Brown bold error bars with caps are for MSPs, where δDM_orb is larger than σ_ΔDM. The grey area shows the region where .