E Castor (Alpha Geminorum) is a multiple star system.
A, B, & C are each spectroscopic binaries.
C is also variable "YY Geminorum".
Jim Kaler - Castor
D is part of this system according to the WDS Catalog.
Washington Double Star Catalog

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NOTE: C & D are enhanced in this photo while A & B are not,
avoiding over-exposure of the much brighter components.