The Pulsating White Dwarf G117-B15A: Still the Most Stable Optical Clock Known
The Pulsating White Dwarf G117-B15A: Still the Most Stable Optical Clock Known
The pulsating hydrogen atmosphere white dwarf star G 117-B15A has been observed since 1974. Its main pulsation period at 215.19738823(63) s, observed in optical light curves, varies by only (5.12+/-0.82)x10^{-15} s/s and shows no glitches, as pulsars do. The observed rate of period change corresponds to a change of the …