These are spectral class B stars that show prominent emission lines of hydrogen in their spectrum. Most of the Be stars in the cluster lie between spectral class B0 and B3.[2] A candidate member of the cluster, LS I +61° 235, is a Be star with an X-ray binary component that has a period of about three years.[5] There are at least five blue stragglers in the cluster. These are stars that formed by the merger of two other stars.[3] Two of the cluster's star systems are likely eclipsing binaries with periods of 0.6 and 1.03 days.[6] NGC 663 also has two red supergiant stars, both located on its periphery[7][8]