4
I have had a core file or two appear in the past and deleted them with no problem but you'll probably want to double check with your host or someone with more experience than me.
I do know that mine appeared after I had some sort of problem or error and they were on a site using Smarty (non XOOPS) - though I don't know if having Smarty was a coincidence.
C
[size=x-small]Working sites:
XOOPS 2.0.16 PHP 5.2.2, MySQL 5.0.24a-standard-log, Apache/2.0.54 (Unix)
XOOPS 2.2.4, PHP 4.3.10, MySQL 3.23.58, Apache/1.3.33 (Unix)[/size]