SIMBAD references

At present three pulsars are known which clearly show a strongly increasing degree of circular polarization with frequency. As this is accompanied by a smooth decrease of linear polarization, we investigate, if this observation can be explained through a propagation mode transition from linear to circular polarization. Using a previously published model we find that the rate of change of the polarization types with frequency is well consistent with the theory. The small number of only three objects does of course not allow to draw a general conclusion for the whole sample of pulsars. However we show that the very unusual behavior of these three objects can be well explained with this model and we discuss, why only so few such objects are presently known.