| Abstract | 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. |
