TOWARD ESTIMATION OF H(0) FROM VLBI OBSERVATIONS OF THE GRAVITATIONALLENS SYSTEM-0957+561

We have analyzed two epochs of 6 cm VLBI observations of the two brigh test images, A and B, of the gravitationally lensed quasar 0957+561 in a search for correlated structural evolution from which to estimate D ELTAtau(BA) the difference in propagation time for light traveling bet ween the quasar and Earth along the paths corresponding to these two i mages. However, we detected only marginal increases in the separation between the core and the innermost jet component in each image over th e 2 year interval 1987-1989. Because monitoring image brightness varia tions has since led to apparently reliable estimates of DELTAtau(BA), we intend to use spatial variations in the relative magnification fiel d across the extent of the images to constrain further the mass distri bution of the lens, which is now the principal contributor to the unce rtainty in estimates of H0 from this lens system.