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NGC 6388 and NGC 6441 are two massive Galactic bulge globular clusters whichshare many properties, including the presence of an extended horizontal branch(HB), quite unexpected because of their high metal content. In this paper weuse HST's WFPC2, ACS, and WFC3 images and present a broad multicolor study oftheir stellar content, covering all main evolutionary branches. Thecolor-magnitude diagrams (CMDs) give compelling evidence that both clustershost at least two stellar populations, which manifest themselves in differentways. 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 inour data set; its red HB is also split into two branches. NGC 6441 has a splitMS, but only an indication of two SGB populations, while the RGB clearly splitsin two from the SGB level upward, and no red HB structure. The multicoloranalysis of the CMDs confirms that the He difference between the two mainstellar populations in the two clusters must be similar. This isobservationally supported by the HB morphology, but also confirmed by the colordistribution of the stars in the MS optical band CMDs. However, a MS splitbecomes evident in NGC 6441 using UV colors, but not in NGC 6388, indicatingthat the chemical patterns of the different populations are different in thetwo clusters, with C, N, O abundance differences likely playing a major role.We also analyze the radial distribution of the two populations.

The intriguing stellar populations in the globular clusters NGC 6388 and NGC 6441