How Google is managing authentication between multiple domains?

Question

I think everyone here would be knowing that, if I am logged in any of the google's website(i.e www.orkut.com), I don't need to authenticate again for www.gmail.com.
How google is managing this. I think, it would be a cookie stored on user's browser. But, if it is a cookie, how different domains are able to read this cookie. What do you say.

Answer 1

They use a system simmlar to OpenID

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