We present flux-ratio curves of the fold and cusp (i.e. close multiple) images of six JVAS/CLASS gravitational lens systems. The data were obtained over a period of 8.5 months in 2001 with the Multi-Element Radio-Linked Interferometer Network (MERLIN) at 5–GHz with 50 mas resolution, as part of a MERLIN Key-Project. Even though the time delays between the fold and cusp images are small (�1d) compared to the time-scale of intrinsic source variability, all six – 2 – lens systems show evidence that suggests the presence of extrinsic variability. In particular, the cusp images of B2045+265 – regarded as the strongest case of the violation of the cusp relation (i.e. the sum of the magnifications of the three cusp images add to zero) – show extrinsic variations in their flux-ratios up to ∼40 % peak-to-peak on time scales of several months. Its low Galactic latitude of b ≈ −10 ◦ and a line-of-sight toward the Cygnus superbubble region suggest that Galactic scintillation is the most likely cause. The cusp images of B1422+231 at b ≈ +69 ◦ do not show strong extrinsic variability. Galactic scintillation can therefore cause significant scatter in the cusp and fold relations of some radio lens systems (up to 10 % rms), even though these relations remain violated when averaged over a �1 year time baseline. Subject headings: gravitational lensing — scattering — ISM: general 1
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