V407 Vul ( = RXJ1914.4+2457 ) shows pulsations in X-ray flux on a period of 9.5 minutes , which have been ascribed to accretion onto a magnetic white dwarf , with the X-ray pulses seen as the accreting pole moves into and out of view .
The X-ray flux drops to zero between pulses , and no other periods are seen , suggesting that V407 Vul is a type of system known as a “ polar ” in which the white dwarf has a strong enough field to lock to the orbit of its companion .
If so , then V407 Vul has the shortest orbital period known for any binary star .
However , unlike other polars , V407 Vul shows neither polarization nor line emission .
In this paper we propose that V407 Vul is the first example of a new type of X-ray emitting binary in which the mass transfer stream directly hits a non-magnetic white dwarf as a result of the very compact orbit .
Our model naturally explains the X-ray and optical pulsations , as well as the absence of polarization and line emission .
We show that direct impact will occur for plausible masses of the accreting star and its companion , e.g . M _ { 1 } \approx 0.5 , M _ { 2 } \approx 0.1 \mathrm { M } _ { \odot } .
In our model V407 Vul retains its status as the binary star with the shortest known orbital period , and is therefore a strong source of low-frequency gravitational waves .
V407 Vul is representative of an early phase of the evolution of the AM CVn class of binary stars and will evolve into the normal disc-accretion phase on a timescale of 10 ^ { 6 } to 10 ^ { 7 } \hbox { yr } .
The existence of V407 Vul supports the double-degenerate route for the formation of AM CVn stars .