1999A&A...348..381R

We determined the separations of the radio sources in the triangle formed by the BL Lac objects 1803+784 and 2007+777, and the QSO 1928+738 from intercontinental interferometric observations carried out in November 1991 at the frequencies of 2.3 and 8.4GHz simultaneously. Difference phase-delay astrometry yielded the following separations:
Δα_{(1803+784)–(1928+738)} -1^h 27^m 2.811256s 0.000062s
Δδ_{(1803+784)–(1928+738)} 4° 30' 2.44833" 0.00012"
Δα_{(1928+738)–(2007+777)} -0^h 37^m 42.503305s 0.000033s
Δδ_{(1928+738)–(2007+777)} -3° 54' 41.67756" 0.''00013
Δα_{(2007+777)–(1803+784)} 2^h 4^m 45.314561s 0.000052s
Δδ_{(2007+777)–(1803+784)} -0° 35' 20.77077" 0.00013"
We successfully connected differenced phase delays over 7° on the sky at 8.4GHz at an epoch of maximum solar activity. The effects of the ionosphere on these VLBI data were mostly removed by estimates of the total electron content from observations of GPS satellites. The comparison of the estimated separation of QSO1928+738 and BL2007+777 with previous such estimates obtained from data at different epochs leads us to a particular alignment 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±0.10mas/yr and also suggest an identification for the position of the core of this radio source.