Almost 25,000 light years away, two dead stars orbit one another. Each more massive than our Sun, only 20 km in diameter, and less than five hours per orbit. This unusual pair was discovered by an international team of scientists – including researchers from two MPIs (Gravitational Physics and Radio Astronomy) – and by volunteers from the distributed computing project Einstein@Home. Only 14 similar binary systems are known so far, and the new one also is the most massive of those. Such systems enable some of the most precise tests of Einstein’s theory of general relativity. They also play an important role as potential gravitational-wave sources for the LIGO detectors. Neutron stars are the highly magnetized and extremely dense remnants of supernova explosions. Like a rapidly rotating cosmic lighthouse they emit beams of radio waves into space. If Earth happens to lie along one of the beams, large radio telescopes can detect the neutron star as a pulsating celestial source: a radio pulsar.