Erlang is a dynamically typed concurrent functional language of increasing interest in industry and academy. Official Erlang distributions come equipped with Dialyzer, a useful static analysis tool able to anticipate runtime errors by inferring so-called success types, which are overapproximations to the real semantics of expressions. However, Dialyzer exhibits two main weaknesses: on the practical side, its ability to deal with functions that are typically polymorphic is rather poor; and on the theoretical side, a fully developed theory for its underlying type system –comparable to, say, Hindley-Milner system– does not seem to exist, something that we consider a regrettable circumstance. This work in progress is the starting point of a medium-term project aiming at improving both aspects, so that at its end we should have proposed a full type system able to infer polymorphic success types for Erlang programs, accompanied by solid theoretical foundations including adequateness results for the type system. In this first step we only provide a derivation system of monomorphic success types for Erlang, along with correctness results with respect to a suitable semantics for the language.