auto login of geust

[capture]

hi i am relitivly new to asp.net and have tried this following code to see if the user is allready loged in and is they arn't to log them in as a geust

string geust = "Geust";

username.Text = HttpContext.Current.User.Identity.Name;
if (username.Text != null )
{
FormsAuthentication.Authenticate(geust);
}

i get the error
No overload for method 'Authenticate' takes '1' arguments

anyone any help how to do this

thanks

dan