This star was found to be variable by T. H. E. C. Espin in 1886.[10] The binary nature of this system was reported by W. J. Hussey in 1900, listing an angular separation of 0.22″ along a position angle of 195.2°.[11]L. Campbell used light curves of the star from 1904 to 1921 to find a variability period of 337 days. In 1922, G. Van Biesbroeck showed that only the northward member of the pair is variable.[10]P. W. Merrill in 1923 found a stellar classification of M6e for the variable component at maximum, while the secondary is K0.[12] At minimum, the primary becomes fainter than the secondary in visual wavelengths. This make X Ophiuchi the Mira variable with the lowest maginitude variation known.[7]
By 1959, the orbital arc of the pair had spanned an angle of 50°, providing an estimated orbital period of at least 500 years and a separation of 64 AU. Preliminary orbital models suggested a mass of 0.8 M☉ for the variable component and 1.2 M☉ for the K-type giant.[14] According to a 2014 publication, the total mass of the system is 4.5 M☉, with 2/3 of this mass (or 3 M☉) belonging to the K-type giant, and the remaining 1.5 M☉ belonging to the Mira variable.[7] The latter star showed an age of around five billion years. At that mass, the variable component must have undergone significant mass loss in order to have already evolved away from the main sequence. The K-type component may have received up to 0.1 M☉ from the donor star.[14]
A 2007 model gives an orbital period of about 877 years with an eccentricity (ovalness) of 0.45.[6] The variable component is an oxygen-rich M-type Mira variable.[15] The duration, amplitude, and shape of the light curve is found to vary slightly from cycle to cycle. Some show a hump during the ascending curve.[16] Its estimated mass loss rate is 2.95×10−9M☉·yr−1.[8] In 1992, a strong radio flare event was observed from this system at a frequency of 1,667 MHz.[17][15] This outburst came from an OHmaser in the inner part of a shell surrounding the variable. Water in the shell is being photodissociated to create OH molecules.[18]
^ abcDucati, J. R. (2002), "Catalogue of Stellar Photometry in Johnson's 11-color system.", VizieR Online Data Catalog, CDS/ADC Collection of Electronic Catalogues, 2237, Bibcode:2002yCat.2237....0D.
^Keenan, Philip C.; et al. (1974), "Revised Catalog of Spectra of Mira Variables of Types ME and Se", The Astrophysical Journal Supplement Series, 28: 271, Bibcode:1974ApJS...28..271K, doi:10.1086/190318.
^ abcdLiu, Jiaming; Jiang, B. W.; Li, Aigen; Gao, Jian (April 2017), "On the silicate crystallinities of oxygen-rich evolved stars and their mass-loss rates", Monthly Notices of the Royal Astronomical Society, 466 (2): 1963–1986, arXiv:1612.02115, Bibcode:2017MNRAS.466.1963L, doi:10.1093/mnras/stw3165.
^Hussey, William J. (November 1900), "Observations of one hundred new double stars", Astronomical Journal, 21 (485): 35–39, Bibcode:1900AJ.....21...35H, doi:10.1086/103240. Data for: 198. DM. +8°3780.
^Merrill, P. W. (May 1923), "Spectroscopic observations of both components of the variable double star X Ophiuchi", Astrophysical Journal, 57: 251–256, Bibcode:1923ApJ....57..251M, doi:10.1086/142753.
^Marsakova, V. I.; Andronov, I. L. (1998), Dusek, J. (ed.), "Cycle-to-Cycle Changes of Mira-type Variable X Oph", Proceedings of the 20th Stellar Conference of the Czech and Slovak Astronomical Institutes. 5th - 7th November 1997. Brno, Czech Republic, Brno, p. 130, Bibcode:1998vsr..conf..130M, ISBN80-85882-08-6.
^Le Squeren, A. M.; Etoka, S. (March 1993), Marsden, B. G. (ed.), "X Ophiuchi", IAU Circular, 5724: 1, Bibcode:1993IAUC.5724....1L.
^Etoka, S.; Le Squeren, A. M. (November 1996), "An OH maser flare in the Mira X Ophiuchi", Astronomy and Astrophysics, 315: 134–140, Bibcode:1996A&A...315..134E.
