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Equations of motion of null cosmic strings near black holes, or other massivesources, are solved exactly in the weak field approximation. The stress-energytensor of a null string in a curved spacetime is introduced and used to showhow scattering by black holes transforms linear and angular momenta of thestring. The corresponding recoil effect of a black hole and change of itsangular momentum caused by a null cosmic string are calculated. For a nullstring, its energy $\mu$ per unit length evolves along the null direction ofthe string trajectory. The evolution of $\mu$ is connected with a stringoptical scalar $Z$. Optical properties of null strings are that their energy isconcentrated on caustics, where $Z$ has poles. String parameters $\mu$ and $Z$capture important features of the spacetime where strings move. Explicitdependence of $\mu$ and $Z$ on the strain and Bondi news tensors ofgravitational wave background, mass and angular momentum aspects areestablished, near the future null infinity, up to the 4-th order in expansionin an inverse null parameter in asymptotically flat spacetimes.

Null Cosmic Strings: Scattering by Black Holes, Optics and Spacetime Content