This star was in the head of the traditional Arabic constellationAl Faras al Kamil, the Complete Horse, as opposed toPegasus which is the front half of a horse.[19] TheIAU Working Group on Star Names approved the name Alfarasalkamil for Omicron Andromedae Aa on 8 May 2025 and it is now so entered in the IAU Catalog of Star Names.[18]
Omicron Andromedae is a multiple star containing at least three components. It may consist of two close pairs in a wider orbit, making a four-star system,[16] although the binarity of the primary star is in doubt.[12] This star system has apeculiar velocity of34.5 ± 5.9 km/s, which qualifies it as arunaway star.[16]
The components A and B were first resolved in 1949, when they were reported to be separated by less than 0.1".[21] In 1975 they were separated by 0.375"[13] and by 2014 by only 0.21".[22] An orbit has been derived with a period of 118 years.[9] The companion is 2.3 magnitudes fainter than the primary star.[5]
In 1975, a companion was discovered byspeckle interferometry only 0.05" from component A.[5] Components Aa and Ab orbit every 5.6 years,[11] although the existence of this companion is now doubted.[12]
Aspectroscopic binary in the system was suspected and in 1988 it was confirmed. Although a clear 33.01 day period was seen, it was unclear which component was the pair seen in the spectrum.[13] Eventually, it was settled that component B was a closespectroscopic binary.[11]
Omicron Andromedae is aGamma Cassiopeiae typevariable star and the system's brightness varies from magnitude +3.58 to +3.78. The variable component is the brightest and most massive star in the system, Aa.[3] Omicron Andromedae also shows variations with a period of about a day, similar to aβ Lyrae-type eclipsing variable, but these are thought to be intrinsic to one of the components and not due to eclipses.[6]
The spectrum is predominantly that of a B6giant star, from the brightest component in the system. It is ashell star and the spectrum containsemission lines with variable profiles.[5] Rapid variations in its spectrum have been reported.[24]
Spectral lines similar to an A2 star are also detectable in the spectrum and these are thought to originate in the B component.[5]
^abcdNicolet, B. (1978). "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System".Observatory.Bibcode:1978ppch.book.....N.
^abcSamus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)".VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S.1: B/GCVS.Bibcode:2009yCat....102025S.
^abSlettebak, A (1982). "Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars".Astrophysical Journal Supplement Series.50: 55.Bibcode:1982ApJS...50...55S.doi:10.1086/190820.
^abcZhuchkov, R. Ya; Malogolovets, E. V.; Kiyaeva, O. V.; Orlov, V. V.; Bikmaev, I. F.; Balega, Yu.Yu; Safina, D. I. (2010). "Physical parameters and dynamical properties of the multiple star o and".Astronomy Reports.54 (12):1134–1149.Bibcode:2010ARep...54.1134Z.doi:10.1134/S1063772910120061.S2CID121960504.
^abcdeMaestro, V.; Che, X.; Huber, D.; Ireland, M. J.; Monnier, J. D.; White, T. R.; Kok, Y.; Robertson, J. G.; Schaefer, G. H.; Brummelaar, T. A. T.; Tuthill, P. G. (2013), "Optical interferometry of early-type stars with PAVO@CHARA - I. Fundamental stellar properties",Monthly Notices of the Royal Astronomical Society,434 (2): 1321,arXiv:1306.5937,Bibcode:2013MNRAS.434.1321M,doi:10.1093/mnras/stt1092,S2CID2361434
^Diz, H. R. (December 2024). "Rapid Periodic Spectroscopic Variation of the H-alpha Line of the Be Shell Star omicron Andromedae".The Journal of the American Association of Variable Star Observers.52 (2): 202.Bibcode:2024JAVSO..52..202D.
