F# fibonacci numbers

It's very warm. Here is an F# fibonacci sequence. I am not sure it's really good functional fu to use mutable values.

let sumUp =
    seq {let fib = ref 0
         let fib1 = ref 1
        while true do 
        yield !fib + !fib1
        let oldfib = !fib 
        fib:=!fib1
        fib1:=oldfib + !fib1}
        
(Seq.take 20 sumUp) |> Seq.iter (fun x -> printfn "%d" x)