gregg writes "HM Cancri has been confirmed as a binary system of two white dwarfs orbiting each other so closely that they complete one orbit every 5.4 minutes; they are separated by a mere 8 Earth diameters. '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. '"