28 Cygni

28 Cygni is a binary star in the northern constellation of Cygnus. It is a faint blue-white hued star but visible to the naked eye with an apparent visual magnitude of 4.93. The distance to 28 Cyg, as estimated from its annual parallax shift of $−0.139$, is around 620 light years. It has an absolute magnitude of −2.56, which means that if the star were just 10 pc away it would be brighter than Sirius, the brightest star in the night sky.

This primary object is a B-type main-sequence star with a stellar classification of B2.5 V, per Lesh (1968). Slettebak (1982) found a class of B2 IV(e), which would suggest this is a more evolved subgiant star. It is a Be star, which means the spectrum displays emission lines due a disk of ejected gas in a Keplerian orbit around the star. The star displays short-term variability with two or more periods, and is classified as an SX Arietis variable by Samus et al. (2017). It is spinning rapidly with a projected rotational velocity of 320 km/s; estimated at round 80% of the critical rotation rate. This is giving the star an oblate shape with an equatorial bulge out to 6.5 times the Sun's radius, compared to 5.7 at the poles. The central star is orbited by a secondary companion star, which is a subdwarf O star. After previous failed attempts to find the star, the companion was detected using interferometry, but the orbital parameters are unknown.