Reproduced plot relating the absolute visual magnitude of LMXBs with the parameter Σ = (L_X/L_Edd)^1/2(P/1 h)^2/3 from van Paradijs & McClintock (1994). vPM94 NSXBs are black circles and BHXBs are black X’s, with reference errors on a point in the lower right. We have added the nominal vPM94 relation line in orange. We have also added a sample of known UCXBs from Armas Padilla et al. (2023) in various colors and marker shapes. Estimations for the orbital period of 4U 1556–60 are shown in three blue stars with decreasing opacity, at +0, +0.5, and +1 log Σ respectively.