Covering semigroups

We introduce and study a semigroup structure on the set of irreducible components of the Hurwitz space of marked coverings of a complex projective curve with given Galois group of the coverings and fixed ramification type. As an application, we give new conditions on the ramification type that are sufficient for the irreducibility of the Hurwitz spaces, suggest some bounds on the number of irreducible components under certain more general conditions, and show that the number of irreducible components coincides with the number of topological classes of the coverings if the number of branch points is big enough.