  Vector perturbations sourced by topological defects can generate rotations in
the lensing of background galaxies. This is a potential smoking gun for the
existence of defects since rotation generates a curl-like component in the weak
lensing signal which is not generated by standard density perturbations at
linear order. This rotation signal is calculated as generated by cosmic
strings. Future large scale weak lensing surveys should be able to detect this
signal even for string tensions an order of magnitude lower than current
constraints.
