Lineshapes for composite particles with unstable constituents

We discuss on very general grounds possible lineshapes of composite particles with one unstable constituent. Expressions are derived in a coupled-channel formalism for constituents interacting in an S-wave with no assumption made on the width of one of them. We show how easy-to-use formulas, already existing in the literature, emerge for narrow constituents and identify the parameter that controls the regime of their applicability.