Albireo, also called Beta Cygni, is a binary star found in the Cygnus constellation. It consists of two stars, a yellow giant called Albireo Aa and a blue main sequence star named Albireo Ac. Another blue star named Albireo B could be part of the system.
Albireo
Optical or physical double star?
Regarding the question whether Albireo A and B form an optical or physical double star, the data situation shows an inconsistent picture. In the past, it was assumed that Albireo A and B are a physical double star, since it is very unlikely that two such bright stars (3.1 mag and 5.1 mag) would lie so close together (0.01°) in the sky by pure chance.
Parallaxes of 8.46 ± 0.58 mas (Albireo A) and 8.67 ± 0.65 (Albireo B) mas were measured with Hipparcos (Hipparcos Catalogue, 1997), corresponding to distances of 386 ± 29 Lj and 376 ± 31 Lj, respectively. Thus, within the error bars, a physical double star is conceivable. On the other hand, quite different values for the proper motions of Albireo A (µα = -7.09 ± 0.52 mas/year, µδ = -5.63 ± 0.66 mas/year) and Albireo B (µα = -1.95 ± 0.52 mas/year, µδ = -0.98 ± 0.66 mas/year) were measured. According to modern Gaia measurements (Gaia DR2, 2018), the parallaxes are 9.95 ± 0.60 mas (Albireo A) and 8.38 ± 0.17 mas (Albireo B), resulting in distances of 328 ± 21 Lj and 389 ± 8 Lj, respectively. Consequently, an optical double star would be present. However, the actual parallax inaccuracy is probably higher than the formal error, since Gaia is not designed to measure very bright stars (G < 6) and the close binary nature of component A is not taken into account.
A 2018 short study, after analyzing Hipparcos and Gaia-DR2 data, concludes that Albireo is "most likely" (most probably) an optical binary. However, this does not argue against a common origin of the components, e.g. as members of a disintegrating motion cluster.