No. 48: Nov-Dec 1986
4U1820-30 is an X-ray binary star located in the heart of the globular cluster NGC 6624. Its claim to fame is a very short period of rotation -- only 685 seconds. In just a shade over 11 minutes, a neutron star orbits a white dwarf, according to present thinking. The orbit diameter is only 1/7 the radius of our sun, which implies the stars themselves are also small. 4U1820-30 is the shortest-period binary ever found -- so short that astronomers are looking for other explanations.
Another curious fact mentioned in this item is that X-ray binaries are much more common in globular clusters than elsewhere in the universe. What is so different about globular clusters?
(King, A.R., and Watson, M.G.; "The Shortest Period Binary Star," Nature, 323:105, 1986.)