  We have used WFPC2 to construct B, V color-magnitude diagrams of four
metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC
6441. All four clusters have well populated red horizontal branches (RHB), as
expected for their metallicity. However, NGC 6388 and 6441 also exhibit a
prominent blue HB (BHB) extension, including stars reaching as faint in V as
the turnoff luminosity. This discovery demonstrates directly for the first time
that a major population of hot HB stars can exist in old, metal-rich systems.
This may have important implications for the interpretation of the integrated
spectra of elliptical galaxies.
  The cause of the phenomenon remains uncertain. We examine the possibility
that NGC 6388 and 6441 are older than the other clusters, but a simple
difference in age may not be sufficient to produce the observed distributions
along the HB. The high central densities in NGC 6388 and 6441 suggest that the
existence of the blue HB (BHB) tails might be caused by stellar interactions in
the dense cores of these clusters, which we calculate to have two of the
highest collision rates among globular clusters in the Galaxy. Tidal collisions
might act in various ways to enhance loss of envelope mass, and therefore
populate the blue side of the HB. However, the relative frequency of tidal
collisions does not seem large enough (compared to that of the clusters with
pure RHBs) to account for such a drastic difference in HB morphology. While a
combination of an age difference and dynamical interactions may help, prima
facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue
against dynamical effects playing a role.
