\documentstyle{preprint}
\begin{document}
\title{POST-AGB STARS AS STANDARD EXTRAGALACTIC CANDLES}
\author{Howard E.\ Bond\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD 21218}

\pub{{\em the proceedings of}\\
~\\
The Extragalactic Distance Scale,\\
~\\
{\em held May 1996 at ST ScI, Baltimore, MD}}

\maketitle

\begin{abstract}
Stars evolving off the asymptotic giant branch and passing through
spectral types F and A are excellent candidates for a new extragalactic
standard candle.  These post-ABB (PAGB) stars are the visually brightest
members of Population~II systems.  They should have a narrow luminosity
function, bounded from above by the shorter transition times of more
massive and more luminous remnants, and from below by the core mass
corresponding to the lowest-mass stars that are leaving the main
sequence.

Moreover, the PAGB A-F supergiants are easily recognized because of
their enormous Balmer jumps, and should lie both in ellipticals and the
halos of spirals.  I describe a photometric system that combines the
Gunn $u$~filter (lying below the Balmer jump) with the standard 
Johnson-Kron-Cousins {\it BVI\/} bandpasses, and report a successful search
for PAGB stars in the halo of M31 using this {\it uBVI\/} system.

The zero-point calibration will come from PAGB A and F~stars in Galactic
globular clusters.  Four are presently known, and have a mean $M_V=-3.4$
with a scatter of only 0.2~mag.  Two are in the same cluster, NGC~5986,
and their $V$~magnitudes differ by only 0.09~mag, strongly suggesting a
narrow luminosity function.  Adopting this $M_V$ and calculating the M31
distance from its halo PAGB stars, we exactly reproduce the accepted
value.

Future plans include a {\it uBVI\/} survey of all Milky Way globular clusters
for PAGB stars in order to strengthen the zero-point calibration, and a
survey of Local Group galaxies to check the calibration.  Ultimately we
believe we can reach the Virgo Cluster through a distance ladder with
only three rungs: subdwarf parallaxes, Milky Way globular clusters, and
then directly to Virgo (with {\it HST\/}).

\end{abstract}
\end{document}