Spectroscopic binary in the constellation Cancer
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! style="text-align: center; background-color: #FFFFC0;" colspan="2" | Observation dataEpoch J2000
      Equinox J2000
! style="text-align:left" | Constellation
! style="text-align:left" | Right ascension
! style="text-align:left" | Declination
! style="text-align:left" | Apparent magnitude (V)
! style="background-color: #FFFFC0; text-align: center;" colspan="2"| Characteristics
! style="text-align:left" | Spectral type
! style="text-align:left" | B−V 
! style="text-align:left" | J−H 
! style="text-align:left" | J−K 
! style="text-align:left" | Variable type
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HD 77191 is a spectroscopic binary composed of a Sun-like variable star and a probable red dwarf, located in the zodiac constellation of Cancer. It has the variable-star designation HL Cancri (abbreviated to HL Cnc). With an apparent magnitude of 8.88, it is too faint to be seen by the naked eye but observable using binoculars as a yellow-hued dot of light. It is located at a distance of according to Gaia EDR3 parallax measurements, and is receding farther away from the Sun at a heliocentric radial velocity of 7.10 km/s.
The star is part of the Castor stream, a moving group of young stars that includes some of the brightest stars in the night sky, such as Castor, Fomalhaut, and Vega.
Stellar properties.
The primary star is a G-type main-sequence star with the spectral type G0V, almost identical to the Sun in mass, effective temperature, and metallicity, but approximately 7% smaller in radius. Its spectrum shows clear signs of high stellar activity and a strong lithium doublet spectral line at wavelength 6707.8 Å, indicative of its youth, with an estimated age of . Accordingly, the star displays large starspots, which are responsible for slight variations in its brightness, first discovered in 2000 with a mean amplitude of about 0.025 mag and a period of (which is also the star's rotation period). Hence, the star is classified as a BY Draconis variable.
Data collected by Hipparcos suggested that the star was single, but radial velocity observations via the Coravel spectrograph at the University of Cambridge yielded a 44-day period orbit for a binary companion. By matching the primary's rotational velocities measured through Doppler broadening and its photometric period, the mass of the unseen secondary star is placed at roughly 0.38 M☉, making it likely a red dwarf.
References.
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