pg_temp files in the database dirs

I am running [PostgreSQL 6.5.0 on i686-pc-linux-gnu, compiled by gcc
2.7.2.3]

I looked into the $PGDATA/base/$DBNAME dirs and there are some pg_temp
files.

This would not normaly raise a question, but they are in the order of
100's of megs and there ctime is from May. I have found these in almost
all of my database dirs.

What are these files from? and how does one clean them up? Is this a
normal thing to happen?

Thank you