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HU Delphini
	; A visual band light curve for HU Delphini, plotted from ASAS-SN data. The main plot shows the long-term variability, and the inset plot shows the periodic variability.
Observation data; Epoch J2000 Equinox J2000
Constellation	Delphinus
Right ascension	20h 29m 48.344s
Declination	+09° 41′ 20.25″
Apparent magnitude (V)	13.07
Characteristics
Spectral type	M4.5V
U−B color index	+1.29
B−V color index	+1.64
Variable type	Flare star
Astrometry
Radial velocity (Rv)	-34.24 ± 0.07 km/s
Proper motion (μ)	RA: 673.1 mas/yr ; Dec.: 122.0 mas/yr
Parallax (π)	113.4 ± 0.2 mas
Distance	28.76 ± 0.05 ly ; (8.82 ± 0.02 pc)
Absolute magnitude (MV)	13.46 / 16.73
Orbit
Period (P)	538.6 ± 0.1 d
Semi-major axis (a)	0.1037 ± 0.0005″
Eccentricity (e)	0.558 ± 0.005
Inclination (i)	145.4 ± 0.6°
Longitude of the node (Ω)	102.2 ± 1.1°
Periastron epoch (T)	JD 2451022.7 ± 0.9
Argument of periastron (ω); (secondary)	12.2 ± 1.5°
Semi-amplitude (K1); (primary)	4.09 ± 0.08 km/s
Semi-amplitude (K2); (secondary)	8.54 ± 0.13 km/s
Details
HU Del A
Mass	0.237 ± 0.004 M☉
Metallicity [Fe/H]	0.07 dex
Rotation	0.3085±0.0005
Rotational velocity (v sin i)	35.1±0.2 km/s
HU Del B
Mass	0.114 ± 0.002 M☉
Other designations
	HU Del, GJ 791.2, CCDM J20298+0941AB, LHS 3556, 2MASS J20294834+0941202, G 24-16
Database references
SIMBAD	data
ARICNS	A
	B

HU Delphini, also known as Gliese 791.2, is a star system in the constellation of Delphinus. Its apparent magnitude is 13.07.^[3] With a trigonometric parallax of 113.4 ± 0.2 mas,^[3] it is about 28.76 light-years (8.82 parsecs) away from the Solar System.

HU Delphini is a binary star with a well-defined period of 538.6 days. The orbit has been derived from astrometry as well as through spectral observations, although that has been difficult because of the high projected rotational velocity. It is also fairly eccentric, at 0.558.^[3]

Both stars in the system are red dwarfs. The primary component of the system is only 23.7% as massive as the Sun,^[3] so it is fully convective.^[4] As a result, there are frequent starspots on its surface, especially near poles.^[7] While the normal surface temperature of the primary is 3000 K, the starspots themselves are cooler: only 2700 K. It is also a flare star,^[4] the first flare been detected in August 2000.^[8] The secondary star has a mass of 11.4% that of the Sun.^[3]

References

^ "ASASSN-V J202948.39+094121.2 / HU Del". All Sky Automated Survey for SuperNovae. Retrieved 1 January 2022.
^ ^a ^b Cutri, Roc M.; Skrutskie, Michael F.; Van Dyk, Schuyler D.; Beichman, Charles A.; Carpenter, John M.; Chester, Thomas; Cambresy, Laurent; Evans, Tracey E.; Fowler, John W.; Gizis, John E.; Howard, Elizabeth V.; Huchra, John P.; Jarrett, Thomas H.; Kopan, Eugene L.; Kirkpatrick, J. Davy; Light, Robert M.; Marsh, Kenneth A.; McCallon, Howard L.; Schneider, Stephen E.; Stiening, Rae; Sykes, Matthew J.; Weinberg, Martin D.; Wheaton, William A.; Wheelock, Sherry L.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues. 2246: II/246. Bibcode:2003yCat.2246....0C.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m Benedict, G. F.; Henry, T. J.; Franz, O. G.; McArthur, B. E.; Wasserman, L. H.; Jao, Wei-Chun; Cargile, P. A.; Dieterich, S. B.; Bradley, A. J.; Nelan, E. P.; Whipple, A. L. (2016). "The Solar Neighborhood. XXXVII. The Mass–Luminosity Relation for Main-Sequence M Dwarfs". The Astronomical Journal. 152 (5): 141. arXiv:1608.04775. Bibcode:2016AJ....152..141B. doi:10.3847/0004-6256/152/5/141. S2CID 54029447.
^ ^a ^b ^c Barnes, J. R.; Jeffers, S. V.; Jones, H. R. A.; Pavlenko, Ya. V.; Jenkins, J. S.; Haswell, C. A.; Lohr, M. E. (2015). "Starspot Distributions on Fully Convective M Dwarfs: Implications for Radial Velocity Planet Searches". The Astrophysical Journal. 812 (1): 42. arXiv:1509.05284. Bibcode:2015ApJ...812...42B. doi:10.1088/0004-637X/812/1/42. S2CID 14907824.
^ ^a ^b Nicolet, B. (1978). "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.
^ ^a ^b "GJ 791.2". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 11 June 2017.
^ ^a ^b ^c Barnes, J. R.; Jeffers, S. V.; Haswell, C. A.; Jones, H. R. A.; Shulyak, D.; Pavlenko, Ya. V.; Jenkins, J. S. (2017), "Surprisingly different star-spot distributions on the near equal-mass equal-rotation-rate stars in the M dwarf binary GJ 65 AB", Monthly Notices of the Royal Astronomical Society, 471: 811–823, arXiv:1706.03979, Bibcode:2017MNRAS.471..811B, doi:10.1093/mnras/stx1482
^ Natsvlishvili R. Sh., Karapetian A. A. A possible flare of a red dwarf HU Del