cookie between domains

[piti]

hi

i have a strange problem with cookies
what i need is following:
when a user visits one of our sites a cookies is drooped on his computer
and when he makes a reservation on another site i want to read the cookie and set a bit saying that he has visited the other site
as far as i know cookies are bound to domain where they are written, so i have tried several things to make it readable on the reservation site but none of them works
i can do that by opening a new window, but i don't want that the user sees something new is opening
do you have any idea how to do that the cookie way or how to remember that the user has visited the non reservation site
thanx a lot