A highly magnetized and rapidly rotating white dwarf as small as the Moon.

White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf.

General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system.

General relativity predicts that short-orbital-period binaries emit considerable amounts of gravitational radiation. The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect tens of thousands of such systems but few have been identified, of which only one is eclipsing-the double-white-dwarf binary SDSS J065133.338+284423.