secure and non-secure pages

[rb]

I'm writing an application that should have few pages [https] secure
(signin, account management etc) and a bunch of pages that are
"public" (http, non-secure).
I thought that because the way certificates work (or so I think) that
would have to setup 2 sites, www and secure, with certificate
assigned to the secure site. The thing I'm sorely missing is how to
pass (and keep) authentication (cookie?) between 2 sites? Or is there
a better way of accomplishing the same?

Thanks.

rb