  In the gravitational lens system B1600+434 the brighter image, A, is known to
show rapid variability which is not detected in the weaker image, B (Koopmans &
de Bruyn 2000). Since correlated variability is one of the fundamental
properties of gravitational lensing, it has been proposed that image A is
microlensed by stars in the halo of the lensing galaxy (Koopmans & de Bruyn
2000). We present VLBA observations of B1600+434 at 15 GHz with a resolution of
0.5 milliarcsec to determine the source structure at high spatial resolution.
The surface brightness of the images are significantly different, with image A
being more compact. This is in apparent contradiction with the required
property of gravitational lensing that surface brightness be preserved. Our
results suggest that both the lensed images may show two-sided elongation at
this resolution, a morphology which does not necessarily favour superluminal
motion. Instead these data may suggest that image B is scatter-broadened at the
lens so that its size is larger than that of A, and hence scintillates less
than image A.
