An analysis of the m = 0 magnetic perturbation in the vacuum region of anRFP device is carried out using toroidal flux and toroidal fieldmeasurements. This perturbation is associated to the locked-mode phenomenonwhich is a coherent superimposition of modes, mainly m = 0,1, locked inphase together. The m = 1 distortion of the magnetic surface is thedominant effect and was considered in a previous analysis. The purpose ofthis work is to study the m = 0 component using the same technique. Theexperimental data are elaborated with simple analytical expressions derivedfrom the vacuum Maxwell equations and the general contravariantrepresentation of the magnetic field in terms of the flux function. Them = 0 geometrical distortion of the magnetic surfaces is reconstructed. Itsdependence on the plasma current and the impact on the plasma wallinteraction are discussed.