select p.spid from v$session s join v$process p on(s.paddr=p.addr) where s.sid=42;
But if you kill a session with 'alter system kill session ...' the link between these views are broken because the value in v$session.addr changes. In order to look for these orphan processes run this query:
select spid, program from v$process where program!= 'PSEUDO' and addr not in (select paddr from v$session) and addr not in (select paddr from v$bgprocess) and addr not in (select paddr from v$shared_server);
You may check with OS tools like ps on Linux to see that these are indeed dead processes or with strace to see what they are doing and eventually kill them.