Perturbations of the electron density and the poloidal magnetic field induced by sawtooth instabilities are studied using interferometric and polarimetric data. The change in the safety factor profile at a sawtooth collapse is deduced. It is found that sawteeth do not lead to complete reconnection of the poloidal flux since ( Delta q0)collapse<1-q0. A comparison of the rate of change of q0 between sawteeth with field diffusion calculations assuming neo-classical resistivity also suggests that complete reconnection does not take place.