SIMBAD references

We report our measurement of the rate of change of period with time (P{dot}) 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 σ measurement P{dot}_observed=(4.27±0.80)x10^–15 s/s. Taking into account the proper-motion effect of P{dot}_proper=(7.0±2.0)x10^–16 s/s, we obtain a rate of change of period with time of P{dot}=(3.57±0.82)x10^–15 s/s. 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.