High precision difference astrometry applied to the triplet of S5 radio sources B1803+784/Q1928+738/B2007+777

We determined the separations of the radio sources in the triangle formed b y the BL Lac objects 1803+784 and 2007+777, and the QSO 1928+738 from inter continental interferometric observations carried out in November 1991 at th e frequencies of 2.3 and 8.4 GHz simultaneously. Difference phase-delay ast rometry yielded the following separations: Delta alpha((1803+784)-(1928+738))=-l(h)27(m) 2.(s)811256+/-0.(s)000062 Delta delta((1803+784)-(1928+738))=4 degrees 30' 2." 44833+/-0." 00012 Delta alpha((1928+738)-(2007+777))=-0(h)37(m)42.(s)503305+/-0.(s)000033 Delta delta((1928+738)-(2007+777))=-3 degrees 54'41." 67756+/-0." 00013 Delta alpha((2007+777)-(1803+784))=2(h) 4(m)45.(s)314561+/-0.(s)000052 Delta delta((2007+777)-(1803+784))=-0 degrees 35'20." 77077+/-0." 00013 We successfully connected differenced phase delays over 7 degrees on the sk y at 8.4 GHz at an epoch of maximum solar activity. The effects of the iono sphere on these VLBI data were mostly removed by estimates of the total ele ctron content from observations of GPS satellites. The comparison of the es timated separation of QSO 1928+738 and BL 2007+777 with previous such estim ates obtained from data at different epochs leads us to a particular alignm ent of the maps of QSO 1928+738 at different epochs relative to those of BL 2007+777, although with significant uncertainty. For this alignment, that the jet components of QSO 1928+738 show a mean proper motion of 0.32+/-0.10 mas/yr and also suggest an identification for the position of the core of this radio source.