title: Evolutionary Timescale of the Pulsating White Dwarf G117-B15A: The Most Stable Optical Clock Known.
abstract: 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, P&d2;=&parl0;2.3+/-1.4&parr0;x10-15 s s-1, it is already constraining the evolutionary timescale of this cooling white dwarf star.
authors.initials: SO
authors.lastname: Kepler
authors.initials: A
authors.lastname: Mukadam
authors.initials: DE
authors.lastname: Winget
authors.initials: RE
authors.lastname: Nather
authors.initials: TS
authors.lastname: Metcalfe
authors.initials: MD
authors.lastname: Reed
authors.initials: SD
authors.lastname: Kawaler
authors.initials: PA
authors.lastname: Bradley