Is it possible to pass forms authentication credentials directly to a web service

Question

Using ASP.NET 2.0.

I'm using Forms Authentication on two sites. The two sites have the same machine key and thus it is possible to log into one site and go directly to a URL on the other site without the need to log in again.

However, I now wish to call a web service on the other site from the code behind on the current site. I need to pass the current credentials but can't find a way of doing it.