protected void Login1_Authenticate(object sender, AuthenticateEventArgs e)
{
//Controllo la login nel DB preesistente 
/*
..
*/
e.Autenticated=true;
FormsAuthentication.RedirectFromLoginPage("pippo", true);
}

 protected void LoginButton_Click(object sender, EventArgs e)
    {
      
        string pwd = Login1.Password.Trim();
        string usr = Login1.UserName.Trim();

     
        if (pwd != "")
        {
            bool auth= AUTH.logged(usr, pwd);
            //AUTH.logged torna true se la login e' andata a buon fine
            
            if(auth)
            {

    //Salvo nella Session il vero UserName e la Password 
                Session["login"] = usr;
                Session["password"] = pwd;
               
                //Uso la login generica per avere la Roles giusta.
                if(Membership.ValidateUser("pippo", "passwordpippo$"))
                  FormsAuthentication.RedirectFromLoginPage("pippo", Login1.RememberMeSet);
            }
        }
        
    }

<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
    <system.web>
        <authorization>
            <allow roles="pippo" />
            <deny users="*" />
            <deny users="?" />
        </authorization>
    </system.web>
</configuration>