IMAP behaving strange

[Marvin]

Hi there,

I setup Gnus to read IMAP as follows
(setq gnus-select-method '(nnimap "mail"
                           (nnimap-address "imap.ece.rochester.edu")
                           (nnimap-list-pattern ("INBOX" "mail/*"
"Mail/*" "INBOX.*"))

))

Which prompt me for my username and password, as I expected; however,
when I type  ^ to view the mailboxes I am prompted for username and
passwd again (creating two instances) is this normal or am I doing
something silly ?

thanks
MMD