In a use case diagram, how are external systems represented?

In a UML Use Case Diagram, the entities that interact with the system being modeled (the System Boundary) are called Actors. An Actor represents a role played by a user, piece of hardware, or another system. External systems that exchange information with the system being defined are always represented as an actor outside the system boundary, just like human users. This clearly separates the responsibilities of the system under study from all entities and systems that lie outside its scope.

(Reference: BCS Requirements Engineering – Use Case Modelling)