  A new single pulse behaviour has been identified in two pulsars, B0919+06 and
B1859+07. Normally both starts emit bright subpulses in a region near the
trailing edge of heir profile. However, occasionally these stars exhibit
"events" wherein the emission longitude gradually decreases by about their
profile width, remains in this position for typically tens of pulses and then
gradually returns over a few pulses to the usual longitude. The effect bears
some resemblance to a profile "mode change", but here the effect is gradual and
episodic. When the separated profiles of the normal and "event" emission, they
reveal a broad and complex profile structure in each pulsar - but one which can
probably be understood correctly in terms of a single conical beam. Possibly
the effect entails an extreme example of "absorption"-induced profile
asymmetry, as suspected in other pulsars. Alternatively, shifting sources of
illumination within the pulsar beam may be responsible.
