An international team of astronomers, including Professor Tom Marsh and Dr Danny Steeghs from the University of Warwick, have shown that the two stars in the binary HM  Cancri definitely revolve around each other in a mere 5.4 minutes. This makes HM Cancri the binary star with by far the shortest known orbital period.  It is also the smallest known binary. The binary system is no larger than 8 times the diameter of the Earth which is the equivalent of no more than a quarter of the  distance from the Earth to the Moon.

The binary system consists of two white dwarfs. These are the burnt- out cinders of stars such as our Sun, and contain a highly condensed form of  helium, carbon and oxygen. The two white dwarfs in HM Cancri are so close  together that mass is flowing from one star to the other. HM Cancri was first noticed as an X-ray source in 1999 showing a 5.4 minutes periodicity but for a long  time it has remained unclear whether this period also indicated the actual orbital period  of the system. It was so short that astronomers were reluctant to accept  the possibility without solid proof.

The team of astronomers, led by Dr Gijs Roelofs of the Harvard-Smithsonian  Center of Astrophysics, and including Professor Tom Marsh and Dr Danny Steeghs at the University of Warwick in the UK, have now used the world's largest telescope, the Keck  telescope on Hawaii, to prove that the 5.4 minute period is indeed the binary  period of the system. This has been done by detecting the velocity variations  in the spectral lines in the light of HM Cancri. These velocity variations are induced by the Doppler effect, caused by the orbital motion of the two stars  revolving around each other. The Doppler effect causes the lines to  periodically shift from blue to red and back.

The observations of HM Cancri were an ultimate challenge due to the extremely short period that needed to be resolved and the faintness of the binary  system. At a distance of close to 16,000 light years from Earth, the binary  shines at a brightness no more than one millionth of the faintest stars visible to the naked eye.

Professor Tom Marsh from the University  of Warwick said; “This is an intriguing system in a number of ways: it has an extremely short period; mass flows from one star and crashes down onto the equator of the other in a region comparable in size to the English Midlands where it liberates more than the Sun's entire power in X-rays. It could also be a strong emitter of gravitational waves which may one day be detected from this type of star system.”

Dr Danny Steeghs of the University of Warwick, said " A few years ago we proposed that HM Cancri was indeed an interacting binary  consisting of two white dwarfs and that the 5.4 minute period was the orbital  period. It is very gratifying to see this model confirmed by our observations,  especially since earlier attempts had been thwarted by bad weather."

The article describing the observations of HM Cancri entitled Spectroscopic Evidence For a 5.4 Minute Orbital Period in HM Cancri will be published in the  Astrophysical Journal Letters of March 10, 2010

"This type of observations is really at the limit of what is currently  possible. Not only does one need the biggest telescopes in the world, but they  also have to be equipped with the best instruments available", explains Professor  Paul Groot of the Radboud University Nijmegen in the Netherlands.

"The binary HM Cancri is a real challenge for our understanding of stellar  and binary evolution," adds Dr Gijs Nelemans of the Radboud University."We  know the system must have come from two normal stars that somehow spiralled  together in two earlier episodes of mass transfer, but the physics of this  process is very poorly known. The system is also a big opportunity for general relativity. It must be one of the most copious emitters of gravitational  waves. These distortions of space-time we hope to detect directly with the  future LISA satellite, and HM Cancri will be a cornerstone system for this  mission."

For further information please  contact:

Professor  Tom Marsh, Department of Physics , University of Warwick  
+44 (0)24765 74739 t.r.marsh@warwick.ac.uk

Or Dr Danny Steeghs, Department of Physics , University of Warwick  
Tel: +44 (0)247 657 3873 d.t.h.steeghs@warwick.ac.uk

Peter Dunn, Head of Communications
Communications Office, University of Warwick,  
+44 (0)24 76 523708 or +44 (0)7767 655860
email: p.j.dunn@warwick.ac.uk

Professor Paul Groot  and Dr Gijs Nelemans
Radboud Universiteit Nijmegen
Paul Groot: +31644646290 pgroot@astro.ru.nl
Gijs Nelemans: +31645120189 nelemans@astro.ru.nl

Full paper at: http://www.iop.org/EJ/abstract/2041-8205/711/2/L138

PR20 9^th March 2010