Problems with navigation after log in

Hi all,

I'm struggling about the following login issue:
I have to sites site A and site B.
One link "login" on site A navigates me to site B. On site B I enter
username and password. If I write a "redirect_to :back" then I'll be
forwarded from site B to site B. But I want to redirect after login from
Site B to Site A. Does anyone know how to manage that?