RX J0806.3+1527  ! style="background-color: #FFFFC0;" colspan="2" | Binary orbit  RX J0806.3+1527 or HM Cancri (sometimes shortened to "HM Cnc" or "J0806" after establishing identity) is an X-ray binary star system about away. It comprises two dense white dwarfs orbiting each other once every 321.5 seconds (in this system the "year" duration is of only 5.4 minutes), at an estimated distance of only apart (about 1/5 the distance between the Earth and the Moon). The two stars orbit each other at speeds in excess of . The stars are estimated to be about half as massive as the Sun. Like typical white dwarfs, they are extremely dense, being composed of degenerate matter, and so have radii on the order of the Earth's radius. Astronomers believe that the two stars will eventually merge, based on data from many X-ray satellites, such as Chandra X-Ray Observatory, XMM-Newton and the Swift Gamma-Ray Burst Mission. These data show that the orbital period of the two stars is steadily decreasing at a rate of 1.2 milliseconds per year as they thus are getting closer by approximately per day. At this rate, they can be expected to merge in approximately 340,000 years. With a revolution period of 5.4 minutes, RX J0806.3+1527 is the shortest orbital period binary system currently known. As RX J0806.3+1527 is a pair of white dwarfs, it has a relatively low optical luminosity. Scientists discovered periodic spikes occurring every 321.5 s by looking at the source in the X-rays. In fact, the 321.5 s modulation of RX J0806.3+1527 was discovered serendipitously in 1999 thanks to the ROSAT mission working in the X-ray band. Optical follow-up observations with the ESO Very Large Telescope (VLT), Telescopio Nazionale Galileo (TNG) and Nordic Optical Telescope (NOT) allowed the counterpart to be identified, a relatively dim (20.7 magnitude in the B filter) object which shows an optical modulation at the same period detected in the X-ray band. The optical monitoring of the counterpart of RX J0806.3+1527 during 2001-2004 clearly shows that the period is decreasing at a rate of about 1/1000 s each year.