We report our measurement of the rate of change of period with time ( \dot { P } ) 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 \sigma measurement \dot { P } _ { \mathrm { observed } } = ( 4.27 \pm 0.80 ) \times 10 ^ { -15 } s / s .
Taking into account the proper-motion effect of \dot { P } _ { \mathrm { proper } } = ( 7.0 \pm 2.0 ) \times 10 ^ { -16 } s / s , we obtain a rate of change of period with time of \dot { P } = ( 3.57 \pm 0.82 ) \times 10 ^ { -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 .