Confusion on how a user is connecting as sysoper
I recently inherited the responsibilities for an Oracle database version 9.0.1 where there is a user, I'll call him BUD, set up to start and stop the database for backups. A script is run where a sqlplus session is run and with BUD logged in as sysoper. I've confirmed through the audit files that it works since I see a nightly file noting that BUD logged in as sysoper and issued a shutdown or startup command.
What I'm confused about is that if I try to interactively log into a sqlplus session with BUD as sysoper, I get the ORA-01031 error. Also, a query of the v$pwfile_users only shows SYS and INTERNAL.