Immersions and folds in string theories of gauge fields

A two-dimensional string model with dynamical cancellation of folds is considered. The action of the model contains the self-intersection number which is defined for surfaces immersed into 4D targets. The two additional variables are not dynamical and live on a compact manifold. In this sense the model is a compactification of a 4D theory. The cancellation forces the string $\th$ angle to be equal $\pi$. Candidates for string states are constructed. Some mathematical background is given.