[Dovecot] NO: Unsupported authentication mechanism

[Steve [Dovecot]]

I'm happily using dovecot as my IMAP server and accessing mails using 
Thunderbird... but I want to use the Net::IMAP facilities in the Ruby 
standard library to write some scripts to automate various spam-related 
tasks & housekeeping.

My Ruby script:

    require "net/imap"
    imap = Net::IMAP.new('localhost')
    imap.authenticate('LOGIN','myuser','mypassword')
    # More stuff using imap...

 From the documentation LOGIN uses plaintext authentication - which I 
understand is always acceptable for Dovecot when connecting from 
localhost.  On the 3rd line I get a "NO response" exception raised 
(indicating that the IMAP server replied "NO") and the error message
        "Net::IMAP::NoResponseError: Unsupported authentication mechanism"
If I change 'LOGIN' to 'CRAM-MD5' (the other value supported by 
Net::IMAP) then I get a similar (but not identical) error:
        "Net::IMAP::NoResponseError: Authentication failed: Unsupported 
authentication mechanism"
If I use anything other than either 'LOGIN' or 'CRAM-MD5' I get an error 
from the Net::IMAP library saying "Unknown auth type" - which is reasonable.

N.B. I've tried both where 'myuser' and 'mypassword' are valid - and 
not... this doesn't affect the error messages.

There doesn't seem to be anything of interest in /var/log/messages...

Can anyone tell me what is going wrong, and how to resolve this?