Authenticating happening on another box.

I hope I can pose this question sensibly. I'm working on an application that will supply somewhat sensitive information to users that are authenticated against another server, and I'd like to know how this is possible: The user request a "home" page that will sit on an NT/IIS /ASP server, and they will be authenticated at that time (I don't know anything about IIS, so the details are unknown to me). This "home" page will provide a URL to pages on my server (which runs UNIX/APACHE/PHP3). Since the user was authenticated on the IIS server, I wondering if there is any way for the USER_AGENT to automatically be authenticated on my server (sort of a hand-off of the authentication between servers) , without the necessity of re-entering a userid and password. Security for this application is important (we're behind a firewall) but not crucial. I'm just trying to come up with something reasonable. Has anyone done this before? Thanks in advance. bumbershoot@centurytel.net