We present results from lambda 3.6 cm observations of the quasar pair 1038528 A and B, made in 1995 using the VLBA together with the Effelsberg 100m
telescope. We describe the use of a phase-referencing technique to measure
the astrometric separation between the quasars. We also introduce a new dat
a analysis method - "hybrid double mapping" - which preserves the relative
astrometric information in a single VLBI hybrid map for close source pairs.
We combine our measurements with those from three previous epochs, the ear
liest in 1981. Our new observations confirm the evolution within the struct
ure of quasar B, previously proposed to explain the measured change in the
relative separation of the pair. Our upper bound for any systematic proper
motion between the mass centres of quasars A and B is 10 mu as yr(-1). This
is set by the limited precision in defining the reference points in the qu
asars at different epochs and by possible instabilities of the source "core
" locations. A separate analysis enables us to put more stringent upper lim
its to any core motions along the two source axes.