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We use results of a survey for low-surface-gravity stars in Galactic (andLMC) globular clusters to show that "yellow" post-asymptotic-giant-branch(yPAGB) stars are likely to be excellent extragalactic standard candles,capable of producing distances to early-type galaxies that are accurate to afew percent. We show that the mean bolometric magnitude of the 10 known yPAGBstars in globular clusters is <Mbol> = -3.38 +/- 0.03, a value that is ~0.2 magbrighter than that predicted from the latest post-horizontal-branchevolutionary tracks. More importantly, we show that the observed dispersion inthe distribution is only 0.10 mag, i.e., smaller than the scatter forindividual Cepheids. We describe the physics that can produce such a smalldispersion, and show that, if one restricts surveys to the color range 0 <(B-V)0 < 0.5, then samples of non-variable yPAGB stars can be identified quiteeasily with a minimum of contamination. The bright absolute V magnitudes ofthese stars (<Mv> = -3.37) make them, by far, the visually brightest objects inold stellar populations and ideal Population II standard candles formeasurements out to ~10 Mpc with current instrumentation. A Hubble SpaceTelescope survey in the halos of galaxies in the M81 and Sculptor groups couldtherefore serve as an effective cross-check on both the Cepheid and TRGBdistance scales.

Yellow Post-Asymptotic-Giant-Branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters