Dear categorists, I would like to announce that the following preprint is available from my webpage www.math.upatras.gr/~pkarazer P. Karazeris and J. Velebil, Dense morphisms of monads. Abstract: Given an arbitrary locally finitely presentable category K and finitary monads T and S on K, we characterize monad morphisms \alpha : S --> T with the property that the induced functor \alpha _* : K^T --> K^S between the categories of Eilenberg-Moore algebras is fully faithful. We call such monad morphisms dense and give a characterization of them in the spirit of Beth’s definability theorem: \alpha is a dense monad morphism if and only if every T-operation is explicitly defined using S-operations. We also give a characterization in terms of epimorphic property of \alpha and clarify the connection between various notions of epimorphisms between monads. The above work bears some relation to the question posed by Todd Wilson on implicitly definable operations. The connection though with non-surjective epimorphisms is not pursued here. Best regards, Panagis Karazeris