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Around 20 binary systems, made up of two white dwarfs that orbit very close to each other, have been discovered so far, but the latest, identified by a research team led by Warren Brown (Harvard-Smithsonian Center for Astrophysics) with the Multiple Mirror Telescope of the Whipple Observatory (Mt. Hopkins, Arizona) is quite extraordinary.
The pair consists of two extremely low-mass white dwarfs (ELM WDs for short!), the smaller of which encloses a mass equal to a quarter of the mass of the Sun in an object the size of Neptune, while the other, heavier and denser, is only the size of the Earth despite weighing half a solar mass.
The orbits of the two white dwarfs about their common barycentre is oriented along our line of sight so that there are eclipses every 6.5 minutes, implying an orbital period of 13 minutes. The orbital speed is at least 600 km/s and the distance between the two bodies does not exceed 1/3 of the Earth-Moon distance.
The collapsed stars making up this system are also of particular interest because they will merge within the lifetime of the Universe (something that can only be said of another dozen systems). In fact in only 900 thousand years they will collide with each other and it is thought that this will create an "underluminous" supernova. Results so far seem to indicate that the numbers of white dwarf pairs destined to merge and underluminous supernova are similar, implying that the former do indeed cause the latter.
But the last pair of white dwarfs discovered by Brown and colleagues is also important for another reason: in contrast to what is shown in the illustration, the stars are not exchanging mass, and their decaying orbits are due to the emission of gravitational waves, which carry away energy from the system.
Though we are not yet able to detect gravitational waves directly, the lack of mass transfer between the stars makes it easier to measure, with great precision, the slow reduction in the orbital period, and test the predictions of Einstein's theory of General Relativity.
As soon as the system comes out of conjunction with the Sun, Brown's team will start the necessary timing measurements and within a few months the test should be complete.
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