Young open cluster IC 4996 and its vicinity: multicolor photometry and
  Gaia DR2 astrometry
  The open cluster IC 4996 in Cygnus and its vicinity are investigated by
applying a two-dimensional photometric classification of stars measured in the
Vilnius seven-color photometric system. Cluster members are identified by
applying distances based on the Gaia DR2 parallaxes and the point vector
diagram of the Gaia DR2 proper motions. For some B-type stars, spectroscopic MK
types are also obtained from the Asiago spectra and collected from the
literature. New parameters of the cluster are derived. The interstellar
extinction $A_V$ covers a wide range of values, from 1.3 to 2.4 mag; the mean
value in the central part of the cluster is 1.8 mag. The cluster distance is
1915 $\pm$ 110 pc, and its age is within 8-10 Myr. The cluster exhibits a long
sequence from early-B to G stars, where stars cooler than B8 are in the
pre-main-sequence stage. The plot of extinction versus distance shows a steep
rise of $A_V$ up to 1.6 mag at 700-800 pc, which is probably related to dust
clouds at the edge of the Great Cygnus Rift. The next increase in extinction by
an additional 0.8 mag at $d$ $\geq$ 1.7 kpc is probably related to the
associations Cyg OB1 and Cyg OB3. The cluster IC 4996 does not belong to the
Cyg OB1 association, which is located closer to the Sun, at 1682 $\pm$ 116 pc.
It seems likely that the cluster and the surrounding O-B stars have a common
origin with the nearby association Cyg OB3 since Gaia data show that these
stellar groups are located at a similar distance.
