Red supergiant candidate stars have been found in Ml01 starting at V = 20.3, rather than fainter than V = 21.0 as previously reported. One of the brightest of three such stars is variable, ensuring its membership. Photometry of ∼200 stars, in or near the spiral arms of M 101, has been obtained in B and V magnitudes, giving a color-magnitude diagram similar to that for the brightest stars in M33 but displaced faintward by ∼3.9 mag. New data on three of the brightest blue supergiant irregular variables in Ml01 give an apparent blue modulus of (m - M )ABM101 = 29.2 comparing with the brightest blue irregular variables in M33 and adopting (m - M )ABM33 = 25.35 found earlier for M33 from its Cepheids. The lack of Cepheids in M101 brighter than B = 22.7 requires this value as a firm lower limit to the Ml0l modulus. The new data on the red supergiants require v(1)> = -8.9 and v(3) ≃ -8.7 (depending on membership) for M 101, nearly independent of assumptions on the internal absorption. The previous determination of the Hubble constant of HO = 50 km 5 s⁻¹ Mpc⁻¹ by Sandage and Tammann using red supergiants is unaffected by these new M 101 results if the present interim calibration of Mv(3) = f(MBT) is correct.
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A. Sandage (1983) studied this question.