Forms Authenciation in IFrame IE Problem

I have created an Iframe in an existing web application (non asp.net but uses its own forms authenciation and cookie).  This existing application is not in the same domain as the site being rendered into the IFrame.  When using FireFox everything works fine. When using IE my login does not work.  I enter the user credintals and I end up right back at the login page.  I thought the problem might have something to do with the url parameters that the login control uses but I put a link on the login page to take me directly to my default page with no luck.  My guess is the cookie is not getting properly saved?

Thanks for any help.

 
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