A change in the variability properties of the intraday variable quasar 0917+624

We observed the quasar 0917+624 at three epochs at a wavelength of 6 cm in order to study its short-timescale variability. In December 1997 and February 1999, the source showed its usual characteristics: variations on a 10-15 % level have been found on timescales less than or equal to 1.5 days. In contrast, in September 1998 the source exhibited a monotonic increase of about 7 % during a five-day-observing campaign. Assuming an intrinsic origin of the variations, this corresponds to a change in brightness temperature of at least two orders of magnitude. We discuss shortly several mechanisms which may cause the observed change of the variability properties of 0917+624.