Evolutionary Timescale of the DAV G117-B15A: The Most Stable Optical   Clock Known
  We observe G117-B15A, the most precise optical clock known, to measure the rate of change of the main pulsation period of this blue-edge DAV white dwarf. Even though the obtained value is only within 1 sigma, Pdot = (2.3 +/- 1.4) x 10^{-15} s/s, it is already constraining the evolutionary timescale of this cooling white dwarf star. 
S. O.Kepler, Anjum Mukadam, D. E. Winget, R. E. Nather, T. S.   Metcalfe, M. D. Reed, S. D. Kawaler and Paul A. Bradley