In 1994, astronomers using the German ROSAT space observatory discovered a strong X-ray source in the constellation of Cancer, and noticed that its intensity fell sharply every five minutes. Binary star system are known to emit X-rays, and this prompted Israel and co-workers to study the system – known as RX J0806.3+1527 – more closely, using the Very Large Telescope in Chile and the Italian Telescopio Nazionale Galileo in the Canary Islands.

The larger star in a binary system has the stronger gravitational field, which means that it draws gas and matter from its companion. Astronomers believe that the energy of this infalling material can make the ‘receiving’ star so hot that it emits X-rays. These X-rays are blocked from view each time the ‘donor’ star passes in front of its companion, leading to a sharp dip in the signal.

High-resolution studies of the binary system by Israel’s group showed that the stars are only 80 000 kilometres apart – around a fifth the distance from Earth to the Moon – which means that they must be travelling at around 1000 kilometres per second. The researchers believe the system may be in a brief transition phase of its evolution, because the orbital periods of binary stars are thought to lengthen over time. The stars are probably different-mass white dwarf stars, each about the size of the Earth.

Israel and colleagues think that the stars in binary system J0806.3+1527 could be moving fast enough to generate gravitational waves – ripples in the fabric of space-time that are predicted to arise when a massive object moves through space. The researchers hope these waves will be detected by the planned Laser Interferometer Space Antenna – or LISA – which is due for launch in ten years’ time. So far only indirect evidence for gravitational waves has been found, but astronomers hope that detectors such as VIRGO and LIGO will find direct evidence for them within a decade.

The research is to appear in the journal Astronomy and Astrophysics Letters.