A Van Kampen type obstruction for string graphs

In this paper we prove that a graph is a string graph (the intersection graph of curves in the plane) if and only if it admits a drawing in the plane with certain properties. This also allows us to define an algebraic obstruction, similar to the Van Kampen obstruction to embeddability, which must vanish for every string graph. However, unlike in the case of graph planarity this obstruction is incomplete.