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We have obtained CCD UBV I KC photometry down to V ∼ 22:0 for the open cluster NGC 2309 and its surrounding field. Twenty three probable cluster members have been identified for the first time on the basis of sound photometric criteria. Because NGC 2309 is a relatively poor cluster projected onto a rich star field showing density fluctuations, the frequently used membership criteria based on stellar density profiles or proper motion measurements would not be enough to assess the star membershipdoi:10.1086/651086 fatcat:rpijigzhqreubj6dvssfob6kmy