SIMBAD references

Magnitudes and color indices of 620 stars down to V = 17.5 mag in the eight-color Vilnius + I photometric system are obtained in the area of the open cluster NGC6996 in the North America Nebula. By combining the results of optical photometry and the infrared 2MASS data an increased value of the ratio of total-to-selective extinction is found. For early-type stars it corresponds to R_BV=3.5. Spectral types, interstellar color excesses, extinctions and distances of stars are determined from the photometric data. The plot of extinction vs. distance shows that the extinction exhibits a steep rise at 400 pc reaching 3-4 mag. Forty seven main-sequence stars and three red giants are identified as probable cluster members. The cluster distance is found to be 794 pc by ZAMS fitting and 781 pc by averaging individual distances of the 50 probable cluster members. The cluster stars show a range of interstellar extinction, with an average value of A_V is 1.92 mag. Fitting the main sequence and red giants of the cluster to the isochrones in the M_V vs. (B-V)₀ plane gives an age of 3.5x10⁸ yr. Probably NGC 6996 has no genetic relation to the star-forming region in the North America and Pelican nebulae. About 35 stars in the magnitude range 16-17 in the field of NGC 6996 are suspected to be O-B5 stars belonging to the Perseus spiral arm.