Double white dwarf binaries are a cornerstone population for binary evolution, provide the dominant component in Galactic gravitational wave emission and connect to type Ia supernova pathways. We present GTC/OSIRIS fast spectroscopy of the 9.5-minute variable V407 Vul in order to prove conclusively that it is indeed one of the most compact binaries known and that the 9.5-minute period represents its binary orbit.