While binary stars are common, believed to have formed close together and orbiting each other since birth, it was always thought that if binary stars form too close to each other, they would quickly merge into one single, bigger star.
Observations over the last three decades have found an abundant population of stellar binaries, but none with orbital periods shorter than 5 hours.
However, astronomers using the United Kingdom Infrared Telescope say the four binary systems they found orbit each other in less than 4 hours, with one system orbiting in just 2.5 hours, previously thought impossible.
"It means that we have to rethink how these close-in binaries form and evolve," lead study author Bas Nefs from Leiden Observatory in the Netherlands said.
Writing in the Monthly Notices of the Royal Astronomical Society, the researchers describe investigating binaries of red dwarfs, stars up to 10 times smaller and a thousand times less luminous than the Sun.
"To our complete surprise, we found several red dwarf binaries with orbital periods significantly shorter than the 5 hour cut-off found for Sun-like stars, something previously thought to be impossible," Nefs said.
It is possible intense magnetic field lines radiating out from the companions could apply the brakes to these spinning stars, slowing them down so that they move closer together, researchers said.