A tiny unseen companion, a low-mass UMP star, is particularly unusual.
J1808−5104 is an ultra metal-poor (UMP) star, one that has a logarithmic metallicity [Fe/H] less than −4, or 1/ 10,000 th of the levels in the Sun.
[8] It is a single-lined spectroscopic binary, with radial velocity variations in its spectral absorption lines interpreted as orbital motion of the visible star.
[1] The primary component of the binary star system, 2MASS J18082002−5104378 A, is a subgiant, cooler than the Sun, but larger and more luminous.
[3] The secondary unseen companion, 2MASS J18082002−5104378 B, thought to be a red dwarf,[5] has an orbital period P = 34.757+0.010−0.010 days and a mass of 0.14 M☉.