IC 4756 is a large bright
open cluster in the constellation
Serpens. Known as
Graff's Cluster, it is bright enough to be seen with the naked eye and considered a fine cluster for binoculars or small telescopes.IC 4756 is also known as the Tweedledee Cluster (paired with
NGC 6633 as Tweedledum), also as the Secret Garden Cluster.
Metallicity of IC 4756 is similar to the Sun, at -0.02±0.01 dex.
Stars:
There are some noteworthy stars in the cluster.
HD 172365[ is a likely post-
blue straggler in the IC 4756 that contains a large excess of
lithium.
[HD 172189, also in IC 4756, is an
Algol variable eclipsing binary=10.5px with a 5.70 day period. The primary star in the system is also a Delta Scuti variable, undergoing multiple pulsation frequencies, which, combined with the eclipses, causes the system to vary by around a tenth of a magnitude.
https://en.wikipedia.org/wiki/IC_4756 
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