Abstract
Process types statically ensure the acceptability of all messages sent to an object even if the set of acceptable messages changes dynamically. As proposed so far, process types do not ensure the return of answers; deadlocks may prevent objects from behaving as expected. In this article we propose an object calculus with types distinguishing between obligatory messages (that must be sent, for example, as answer to a question) and optional messages (that can, but need not be sent). A type checker enforces the sending of obligatory messages and ensures that obligatory messages are not suppressed by deadlocks. The proposed type concept supports subtyping.