Class not found exception (org.apache.openjpa.enhance.PersistenceCapable) thrown by WLS 10 client

I am developing an application using WLS 10 When I try to bind (search) to an EJB from a specific jar of my ear application, an exception of type "ClassNotFoundException" of class "org.apache.openjpa.enhance.PersistenceCapable" is thrown on the client

Has anyone found the reason for this workaround?

Thanks in advance