The intriguing stellar populations in the globular clusters NGC 6388 and
  NGC 6441
  NGC 6388 and NGC 6441 are two massive Galactic bulge globular clusters which
share many properties, including the presence of an extended horizontal branch
(HB), quite unexpected because of their high metal content. In this paper we
use HST's WFPC2, ACS, and WFC3 images and present a broad multicolor study of
their stellar content, covering all main evolutionary branches. The
color-magnitude diagrams (CMDs) give compelling evidence that both clusters
host at least two stellar populations, which manifest themselves in different
ways. NGC 6388 has a broadened main sequence (MS), a split sub-giant branch
(SGB), and a split red giant branch (RGB) that becomes evident above the HB in
our data set; its red HB is also split into two branches. NGC 6441 has a split
MS, but only an indication of two SGB populations, while the RGB clearly splits
in two from the SGB level upward, and no red HB structure. The multicolor
analysis of the CMDs confirms that the He difference between the two main
stellar populations in the two clusters must be similar. This is
observationally supported by the HB morphology, but also confirmed by the color
distribution of the stars in the MS optical band CMDs. However, a MS split
becomes evident in NGC 6441 using UV colors, but not in NGC 6388, indicating
that the chemical patterns of the different populations are different in the
two clusters, with C, N, O abundance differences likely playing a major role.
We also analyze the radial distribution of the two populations.
