logout requirement in portals

Hi,

We have a requirement in our portal (using uPortal) where in user SSO into an external application and the application opens up in a new popup window..
now when user clicks logout from the external app the browser shld close automatically..

We are using uPortal for the portal
and for SSO purpose we use some identity management tool (from Jamcracker)..
ideally for SSO we get a html (with all required hidden params) and we post to the html by opening a new popup window.

So at this point of time we loose the control and user gets into the external app (opened up in a new popup window)..

so how do we control that when users clicks logout from the external app the browse shld close itself

Any ideas/suggestions will be greatly appreciated

Thanks