  Cosmic strings were postulated by Kibble in 1976 and, from a theoretical
point of view, their existence finds support in modern superstring theories,
both in compactification models and in theories with extended additional
dimensions. Their eventual discovery would lead to significant advances in both
cosmology and fundamental physics. One of the most effective ways to detect
cosmic strings is through their lensing signatures which appear to be
significantly different from those introduced by standard lenses (id est,
compact clumps of matter). In 2003, the discovery of the peculiar object CSL-1
(Sazhin et al.2003) raised the interest of the physics community since its
morphology and spectral features strongly argued in favour of it being the
first case of gravitational lensing by a cosmic string. In this paper we
provide a detailed description of the expected observational effects of a
cosmic string and show, by means of simulations, the lensing signatures
produced on background galaxies. While high angular resolution images obtained
with HST, revealed that CSL-1 is a pair of interacting ellipticals at redshift
0.46, it represents a useful lesson to plan future surveys.
