1998A&A...336..385G

From two sets of very-long-baseline interferometry (VLBI) observations, one each at 5GHz (1985.77) and at 2.3 and 8.4GHz simultaneously (1988.83), of the quasar 1928+738 phase-referenced to the BLLac object 2007+777, we infer a change in their relative position of 0.74±0.22 milliarcseconds (mas) along a position angle of -9°±25°, where the quoted standard errors account for errors in our models of the geometry of the interferometric array, the atmosphere, and the source structure. This change is interpreted as an (inadvertent) shift of the reference point selected in the structure of 1928+738, due to the presence of an emerging component; different opacity effects in the three observing frequency bands may also be a significant contributor.