星宿五

长蛇座27，又名BD-08 2643，HD 80586、SAO 136768、HR 3709，是长蛇座的一颗恒星，视星等为4.8，位于银经241.08，银纬27.11，其B1900.0坐标为赤经9h 15m 36s，赤纬-9° 27.11′ 53″。

亚恒星伴星

日本冈山天体物理观测所的系外行星搜寻团队于2008年下半年发表关于一系列已经过主序星演化阶段恒星径向速度观测的论文，论文中指出有亚恒星天体和星宿五组成联星系统。该天体的轨道周期约9.3年，但尚未有行星被确认。

27 Hydrae is a triple star system in the equatorial constellation of Hydra, located 222 light years away from the Sun. It is visible to the naked eye as a faint, orange-hued star with a combined apparent visual magnitude of 4.82. The system is moving further from the Earth with a heliocentric radial velocity of +25.6 km/s.

The magnitude 4.91 primary, component A, is an aging giant star with a stellar classification of K0 III. It is a red clump giant, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star is 1.9 billion years old with 2.17 times the mass of the Sun. It has swelled to 11 times the Sun's radius and is radiating 57.5 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,965 K. The star is suspected to host a low-mass companion.

The stellar companions to this star, designated components B and C, lie at an angular separation of 229.10″ from the primary, and form a binary pair with a separation of 9.20″ as of 2015. The brighter member of the pair, component B, is a seventh magnitude F-type main-sequence star with a class of F4 V, while its companion is an eleventh magnitude K-type main-sequence star with a class of K2 V.

Substellar companion candidate

The Okayama Planet Search team published a paper in late 2008 reporting investigations into radial velocity variations observed for a set of evolved stars, showing hints of a substellar companion orbiting the primary member of the wide binary system 27 Hydrae (ADS 7311). A later study revised the minimum mass of this candidate companion to 28 Jupiter masses, indicating that if it exists, it is a brown dwarf rather than a planet.