G117-B15A[1] is a small, well-observed variable white dwarf star of the DAV, or ZZ Ceti, type in the constellation of Leo Minor.
G117-B15A was found to be variable in 1974 by Richer and Ulrych,[2] and this was confirmed in 1976 by McGraw and Robinson.[3] In 1984 it was demonstrated that the star's variability is due to nonradial gravity wave pulsations. As a consequence, its timescale for period change is directly proportional to its cooling timescale, allowing its cooling rate to be measured using astroseismological techniques. Its age is estimated at 400 million years.[4] Its light curve has a dominant period of 215.2 seconds, which is estimated to increase by approximately one second each 14 million years.[5] G117-B15A has been claimed to be the most stable optical clock ever found, much more stable than the ticks of an atomic clock.[6] It is also the first pulsating white dwarf to have its main pulsation mode index identified.[1]
An X-ray source in the constellation Leo Minor is the white dwarf G117-B15A.[7]