The massive star population of M31 is investigated by means of CCD imaging of selected OB associations; the data encompass UBV photometry of stars in four fields which can be used to identify the hottest and most luminous stars as well as to compare the relatively unevolved massive-star content among such associations. Photometry obtained through interference filters is also used to identify WR star candidates in these and in an additional four fields. Color-magnitude diagrams are obtained which indicate the presence of numerous luminous blue stars inside the association boundaries; these are probably O stars and B supergiants. Outside the association boundaries, a background of slightly older stars is noted.
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Massey et al. (1986) studied this question.