Passing login info between domains

[jwocky]

Ok, I know this topic has been discussed a few times, with various solutions discussed but i havent been able to find the right one yet.

Basicly what I have is a site called www.siteX.com.

You can reach siteX.com by typing in siteX.com or siteY.com into your browser (siteY.com is just pointed to siteX.com, its the identical same site)

but obviously you won't be logged in with both instances because the cookies recognize the domain not the site.

What i'd like to acheieve is a way to pass the cookie from siteX.com to siteY.com

What I mean is when you are on siteX.com there is a link to a subforum and the subforum will be a full link to siteY.com (so you are on siteX.com and the link is to http://www.siteY.com/forums/forumdisplayetcetc) and when that link is clicked the system will automatically pass the cookie to the new siteY.com so that the user doesn't have to login to the same exact site again.

Is this at all possible or anyone have any ideas on how to go about this?

THanks!