Manually Log in User with Provider Model

I am using standard provider model. I have a normal login control, and it all works great.
However, sometimes I get the username and password through other methods and want to manually login the user.


In the codebehind I am looking for something like 
   membership.LoginUser(username,password)
or
   My.User.CurrentPrincipal = New iPrincipal(uername,password)
...anything that will allow me to login a user as the current principal, in the code behind with the username and password I have.


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