Originally Posted by Taykadip

Thank you all. A great first experience with Bike Forums.
The derailleur I bought was clearly intended for a 1" tube (25.4mm), no doubt it came off a Schwinn Varsity. The seller did not indicate a tube size, so I assumed 28.6.
And yes, my Gitane seat tube is 28mm, not 28.6, so I will need to use a shim with a 28.6mm FD clamp.