Converse PDL is the extension of propositional dynamic logic with a converse operation on programs. Our main result states that Converse PDL enjoys the (local) Craig Interpolation Property, with respect to both atomic programs and propositional variables. As a corollary we establish the Beth Definability Property for the logic.

Our interpolation proof is based on an adaptation of Maehara’s proof-theoretic method. For this purpose we introduce a sound and complete cyclic sequent system for this logic. This calculus features an analytic cut rule and uses a focus mechanism for recognising successful cycles.