An average value of the total-to-selective-extinction ratio { R _ { V } = 3.8 } { \pm 0.4 } in M31 is obtained by means of two independent methods and by use of the analytical formula of Cardelli , Clayton & Mathis ( 1989 ) .
This result differs from previous determinations as well from the ‘ standard ’ value 3.1 for the Milky Way .
The derived individual extinctions for blue and red luminous stars from the catalogue of Magnier et al . ( 1992 ) are in good agreement with recent estimates for several OB associations in M31 and thus the issue about the assumed optical opacity of the spiral disk still remains open . The presented list of 113 red supergiant candidates in M31 with their extinctions and luminosities contains 60 new objects of this type which are not identified in other publications .
It is supplemented with further 290 stars dereddened on the base of results for their closest neighbors .
The luminosity function of all red supergiant candidates and the percentage of those with progenitors over 20 { M _ { \odot } } suggests that the evolution of massive stars in M31 resembles that in other Local Group galaxies .