We discuss three aspects of post-AGB ( PAGB ) stars in old populations .
( 1 ) HST photometry of the nucleus of the planetary nebula ( PN ) K 648 in the globular cluster ( GC ) M15 implies a mass of 0.60 M _ { \odot } , in contrast to the mean masses of white dwarfs in GCs of \sim 0.5 M _ { \odot } .
This suggests that K 648 is descended from a merged binary , and we infer that single Pop II stars do not produce visible PNe .
( 2 ) Yellow PAGB stars are the visually brightest stars in old populations ( M _ { V } \simeq - 3.3 ) and are easily recognizable because of their large Balmer jumps ; thus they show great promise as a Pop II standard candle .
Two yellow PAGB stars in the GC NGC 5986 have the same V magnitudes to within \pm 0.05 mag , supporting an expected narrow luminosity function .
( 3 ) Using CCD photometry and a u filter lying below the Balmer jump , we have detected yellow PAGB stars in the halo of M31 and in its dwarf elliptical companion NGC 205 .
With the Milky Way zero point , we reproduce the Cepheid distance to M31 , and find that NGC 205 is \sim 100 kpc further away than M31 .
The star counts imply a yellow PAGB lifetime of about 25,000 yr , and their luminosities imply masses near 0.53 M _ { \odot } .