Undesired Incoming* files created by Gnus

[Rodolfo Medina]

I already posted this on gnu.emacs.gnus but got no solution.

Every time Gnus fetches a new e-mail message from my Gmail account, a
strange
file is created in the ~/Mail directory whose name always starts with
`Incoming'.  I.e.: Incoming3141XpI, Incoming3410dCs, etc.  They're
always
copies of the messages stored in the inbox, that I have to delete
manually:
it's bothering.

Why does this happen, and how can I eliminate it?

Thanks for any help
Rodolfo