External Forms Authentication with Sitefinity

Greetings, my issue is very similar to this thread...

http://www.sitefinity.com/devnet/forums/sitefinity/general-discussions/external-authentication-with-sitefinity-5-0.aspx 

...except for a few slight changes. First we have a separate application running on the same machine which is not running inside of sitefinity. This application does its own authentication with a backend database. Then we have another site which does use sitefinity that I need to be able to authenticate with the first application and also be able to provide single sign on as well. I have read numerous posts about using single sign on between two sitefinity sites but have not been able to find any information for an instance similar to this one. I'm hoping that this can be done one way or another. Thanks

Jon