2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/33943We consider strings with the Nambu action as extremal surfaces in a given space-time, thus, we ignore their back reaction. Especially, we look for strings sharing one symmetry with the underlying space-time. If this is a non-null symmetry, the problem of determining the motion of the string can be dimensionally reduced. We get exact solutions for the following cases: straight and circle-like strings in a Friedmann background, straight strings in an anisotropic Kasner background, different types of strings in the metric of a gravitational wave. The solutions will be discussed.13 pages, LaTeX, reprinted from Astron. Nachr. 310 (1989) 103General Relativity and Quantum Cosmologytext