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tokens: as stars #4 and #6 (late-O-type dwarfs, = 0.8 and 0.9 mag, M K = −4.2 and −3.9 mag). In box3, star #5 has similar properties; it is an O9-B type ( = 1.1, M K = −4.1 mag). Star #3 is the main ionizing star since it is a massive O4-6 ( = 1.3 mag) of about 40 M ⊙ and an age of 5–7 Myr (Ekström et al. 2012 ). Figure 8. The gray scale shows a GLIMPSE 8.0 μ m image of GRS G331.31–00.34. Red long-dashed circles mark the candidate clusters projected into this H ii region. The H ii region is shown with a dot-dashed black circle. The three dotted boxes indicate regions plotted in the CMDs of Figure 9 (see text). Observed stars are marked with blue squares (blue stars) and with red triangles (late-type stars). The blue trees are the results of an MST search with a cutoff distance of 40′′ (0.8 pc). Figure 9. J – K s vs. K s diagrams of the three dotted-square regions marked in Figure 8 (see text). Squares mark detected early stars, and asterisks mark detected late-type stars. As in Figure 5 , diamonds indicate the locations of O4, O8, B2, and B5 dwarfs for a distance of 3.9 kpc at = 0.0 and 1.0 mag. Dashed lines connect the values at = 0 and those at = 1.0 mag. In conclusion, naked OB stars are not concentrated, as one would expect for a mass-segregated population, but are detected inside the bubble in two groups, while condensing molecular clumps are currently located on the shell surrounding the bubble. In the direction of Cl63 (AGALG331.3873−0.360), which is on the shell, we detected another early type, star #7, which is likely to be contributing to the heating of the protostar AGALG331.3873−0.360. These findings are in line with the current literature. Indeed, bound stellar clusters are a rarity, and in more than 90% of the cases (infant mortality and cluster dissolution), we expect associations of massive stars. For example, the massive association Cygnus OB2 (about 7 pc in diameter) is at least 5–7 Myr old, is not associated with gas, and is made of discrete clumps of stars (1–2 pc in size, Wright et al. 2014 , 2016 ). The authors conclude that Cygnus OB2 originated with discrete clumps, and there is no trace of dynamical evolution (e.g., mass segregation). Similar evidence is provided for the younger W33 complex by Messineo et al. ( 2015 ), 5 pc in diameter, which is made up of several condensing molecular clumps and has sparsely distributed exciting stars (2–4 Myr). 6. Summary We performed a spectroscopic survey of bright infrared stars in the direction of GRS G331.34−00.36 (S62), GRS G337.92−00.48 (S36), and MCM2005b77, and for the first time we detected massive members in these regions: a. A total number of 27 early-type and 32 late-type stars were detected. b. We confirm one O4-6 star and one B0-5 star as the main sources of ionizing radiation of GRS G331.34−00.36 (S62), at a kinematic distance of 3.9 ± 0.3 kpc. The stars have of 1.3 mag and 1.0 mag. With N lyc = 49 photons s −1 , the massive O4-6 star can alone account for most of the energy