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.
@article{arxiv.astro-ph/0003478,
title = {Evolutionary Timescale of the DAV G117-B15A: The Most Stable Optical Clock Known},
author = {S. O. Kepler and Anjum Mukadam and D. E. Winget and R. E. Nather and T. S. Metcalfe and M. D. Reed and S. D. Kawaler and Paul A. Bradley},
journal= {arXiv preprint arXiv:astro-ph/0003478},
year = {2009}
}