Strings in homogeneous background spacetimes

The string equations of motion for some homogeneous (Kantowski-Sachs, Bianchi type I and Bianchi type IX) background spacetimes are given, and solved explicitly in some simple cases. This is motivated by the recent developments in string cosmology, where it has been shown that, under certain circumstances, such spacetimes appear as string vacua. Both tensile and null strings are considered. Generally, it is much simpler to solve for the null strings since then we deal with the null geodesic equations of general relativity plus some additional constraints. We consider in detail an Ansatz corresponding to circular strings, and we discuss the possibility of using an elliptic-shape string Ansatz in the case of homogeneous (but anisotropic) backgrounds.