v_LSR versus extinction-corrected magnitude K_e for the observed supergiants. Black dots denote the measurements derived in this work, while red dots are used for those taken from N11. The dashed and dot-dashed lines represent the locus that a supergiant population with mean M_K =  −9.3 (that corresponds to the bulk of RSGC3) and M_K =  −8.0 (all RSGs are expected to be brighter than this limit, Wood et al. 1983) would occupy assuming the rotation curve from Reid et al. (2009). This work uses Θ₀ = 254 kms^-1 and R₀ = 8.4 kpc. The dotted lines mark the expected velocities of the 3 kpc arm at l = 28° and l = 29°, according to Dame & Thaddeus (2008).