IM Pegasi (HR 8703, HD 216489, SAO 108231, HIP 112997) is a variablebinary star system estimated to be 329 light years away from earth. With an apparent magnitude of 5.65, it is visible to the naked eye with some effort at sky coordinates RA 22.88397, Dec 16.84126 -- a few degrees WNW of Alpha Pegasi. The wide public awareness of it, however, is due to its use as the guide star for the Gravity Probe Bgeneral relativity experiment. It was chosen for this purpose because its microwave radio emissions are observable with a large radio telescope network on the ground in a such a manner that its precise position can be related by interferometry to distant quasars. The two components of the binary system are: a red giant primary, 1.8 times as massive and 13 times the diameter of the sun and a yellow dwarf secondary similar in size and mass of the sun. They orbit their common barycenter in a period precisely estimated to be 24.64877 +/-0.00003 days.
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