To be able to understand type Ia supernovae, which enable us to measure the expansion of the universe, and the exotic merger population, which can constrain the physics of stars, we first need to find the shortest period binaries.

There are different ways to achieve this. If a system is compact enough that mass is transferred between the stars, it might have an X-ray signature detectable from space. Complementarily, eclipses or other causes of light variability can be detected at optical wavelengths with ground-based instruments on telescopes such as the Isaac Newton Telescope or the William Herschel Telescope.

Efforts in the hunt for the shortest orbital period systems over the last decades have revealed one particular system with the shortest period of them all – HM Cancri. Every 5.4 minutes the two white dwarfs complete one orbital cycle. In other words, HM Cancri completes nearly 100,000 orbits in the time it takes for the Earth to orbit the Sun once. The two white dwarfs are separated by just 1/10th of the radius of the Sun, causing one star to donate mass to the other.

HM Cancri was discovered over 20 years ago from a bright X-ray detection which exhibits a pulsed signal every 5.4 minutes, but discovering the system was only the start. In the years that followed, the orbital decay of the binary was measured with the Nordic Optical Telescope on La Palma and found to indeed show a decreasing orbital period, such that the two stars are drawing closer together with a trajectory dominated by gravitational wave radiation. In recent years, Munday and collaborators have continued to monitor the orbital decay and have found that not only are the stars drawing closer, but the decay is slowing, such that the system is resisting inspiral.