We collected from literature the information about large-separated pairs of QSOs, which however once were suspected as gravitationally lensed system. We discuss some new results on time delay determinations including optical-radio correlations for QSO 0957+561. We considered some possible observational effects of gravitational lensing by a cosmic string. A future international project for observations of the gravitational lens system UM425 is briefly discussed.
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V. L. Oknyanskij (2005) studied this question.