Measuring the Evolution of the Most Stable Optical Clock G 117-B15A

We report our measurement of the rate of change of period with time () for the 215 s periodicity in the pulsating white dwarf G 117-B15A, the most stable optical clock known. After 31 years of observations, we have finally obtained a 4 measurement observed = (4.27 ± 0.80) * 10^-15 s s^-1. Taking into account the proper-motion effect of proper = (7.0 ± 2.0) * 10^-16 s s^-1, we obtain a rate of change of period with time of = (3.57 ± 0.82) * 10^-15 s s^-1. This value is consistent with the cooling rate in our white dwarf models only for cores of C or C/O. With the refinement of the models, the observed rate of period change can be used to accurately measure the ratio of C/O in the core of the white dwarf.