PSR B1534+11 is a system consisting of two neutron stars orbiting each other, one of which is a pulsar with a period of 37.9 milliseconds. Situated approximately 1000 parsecs distant, the system was used to test Albert Einstein's theory of general relativity, validating the system's relativistic parameters to within 0.2% of values predicted by the theory.[42] The X-ray emission from the system has been found to be present when the non-pulsar star intersects the equatorial pulsar wind of the pulsar, and the system's orbit has been found to vary slightly.[43]