We have obtained CCD UBVI_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 membership status if employed independently. We estimated a cluster radius of 7.9' and a radius at half the maximum of the cluster density profile of 1.7'. Based on the best fits of isochrones computed by the Geneva group to the cluster color-magnitude and color-color diagrams, previously shifted by the cluster reddening (E(B-V)=0.32) and distance (d=2.5kpc), we derived the cluster age (250Myr) and metallicity ([Fe/H]~0.0). If an 8.5kpc distance from the Sun to the center of the Galaxy is assumed, then NGC 2309 is found to be located just in front of the Perseus spiral arm.

Cone search capability for table J/PASP/122/288/table2 (UBVI data of stars in the field of NGC 2309)