Log user creation and deletion

Is it possible to log the creation and deletion of PostgreSQL users?
I'm on PostgreSQL 8.4 and am trying to log using this approach:

log_connections = true
log_disconnections = true
log_statement = ddl
log_directory = /var/log/pgsql
logging_collector = true
log_rotation_age = 1440

However, I didn't see anything about user creation/deletion in the
logging documentation. Is it covered under "ddl" or something else?

Thanks,

Mike