IC 4756
Template:Short description {{#invoke:Infobox|infobox}}Template:Template other{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Infobox open cluster with unknown parameter "_VALUE_"|ignoreblank=y| age | alt | absmag_v | appmag_v | caption | constellation | credit | dec | dist_ly | dist_pc | epoch | galaxy | group | image | image_scale | mass_msol | name | names | notes | ra | radius_ly | size_v }}
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.<ref>Template:Cite web</ref>
IC 4756 is also known as the Tweedledee Cluster (paired with NGC 6633 as Tweedledum), also as the Secret Garden Cluster.<ref>Template:Cite book</ref>
Metallicity of IC 4756 is similar to the Sun, at −0.02Template:± dex.<ref>Template:Citation</ref>
It was discovered by Solon Bailey, and independently by Kasimir Graff in 1922.<ref>https://gardenastronomer.com/2025/09/01/ic-4756-graffs-cluster-in-serpens/</ref>
Stars
There are some noteworthy stars in the cluster. HD 172365<ref name="HD172365Simbad">Template:Cite web</ref> is a likely post-blue straggler in the IC 4756 that contains a large excess of lithium.<ref name="Andrievsky">Template:Cite journal</ref> HD 172189, also in IC 4756, is an Algol variable eclipsing binary<ref name="Ibanoglu">Template:Cite journal</ref> 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.<ref name="Costa">Template:Cite journal</ref>