Passing FormsAuthentication cookie to a WCF service

I have a website that talks to a remote WCF web service. Both use the same custom FormsAuthentication Provider. I would like to authenticate with the WCF service impersonating the user currently logged in the site. I already did this manually, using UserName client credentials but I need to know the user password.
So, what works so fart is this: an authenticated user makes a request, I create a Service Client and set his credentials:

Asp.net forms authentication and multiple domains

I have two domains, domain1.com and domain2.com pointing at the same asp.net website which uses asp.net build in form authentication. The problem is that even if the domains point to the same website the user only get authenticated for one domain at a time. So if he uses www.domain1.com first and then visits www.domain2.com it’s the same website in the back but he only is authenticated for www.domain1.com. The same thing happens if he uses www and not www when visiting the sites.