How does Google manage authentication between multiple domains?

Question

I think everyone here will know that if I am registered on any google website (e.g. www.orkut.com), I do not need to authenticate again at www.gmail.com.
How Google manages this. I think it will be a cookie stored in the user's browser. But, if it’s a cookie, how different domains can read this cookie. What do you say?

Answer 1

They use the simmlar system for OpenID

http://www.windley.com/archives/2006/04/how_does_openid.shtml