Spectroscopic and photometric observations have been obtained of the brightest stars in the dwarf irregular galaxies NGC 6822 and IC 1613 in the Local Group. The brightest blue stars have visual luminosities of approximately -8.5 mag and -8.2 mag in NGC 6822 and IC 1613, respectively. The brightest M supergiant occurs at a visual luminosity of about -8 mag in each of these galaxies. These results confirm the proposal by Sandage and Tammann (1974) that the luminosities of the brightest blue stars depend on the luminosity of the parent galaxy while the maximum luminosity of the brightest red stars is independent of galaxy type. A discussion of the relative populations of luminous blue and red stars in the H-R diagrams suggests that a steeper luminosity function is responsible for the small numbers of luminous blue stars in these low-luminosity irregular galaxies when compared with the supergiant populations in the Galaxy and LMC. The luminosity function for massive stars in a galaxy would have to be sufficiently steep to not produce any stars or very few stars greater than 40 solar masses to invalidate the calibration of the brightest red supergiants at a visual luminosity of about -8 mag.
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R. M. Humphreys (1980) studied this question.