Figure 1: Artist's impression of a brown dwarf.
Brown dwarfs are objects that are not massive enough to sustain hydrogen burning in their cores. As a result, brown dwarfs become gradually less luminous as they cool with time. Nevertheless, without additional information, the evolutionary state of a brown dwarf cannot be known because the mass and age of a brown dwarf are degenerate parameters. For example, an old, massive brown dwarf can appear similar to a young, low-mass brown dwarf. However, if a brown dwarf has a companion star, the presence the companion can help break the mass and age degeneracy.
Crepp et al. (2016) present the discovery of a brown dwarf in orbit around a Sun-like star with 0.82 ± 0.04 times the mass and 0.79 ± 0.03 times the radius of the Sun. The star is identified as HD 4747A and it is located ~60 light years away. Combining radial velocity measurements taken over 18 years with astrometric measurements, the brown dwarf around HD 4747A, identified as HD 4747B, is estimated to have ~60.2 times the mass of Jupiter.
Figure 2: Radial velocity measurements indicating the presence of HD 4747B. Crepp et al. (2016)
The average distance of 4747B from HD 4747A is ~16.4 AU and the orbital period of HD 4747B is ~38 years. Also, the eccentricity of the brown dwarf's orbit is estimated to be ~0.74, indicating it is in a rather eccentric orbit. HD 4747A is determined to have an age of roughly 3.3 billion years. Its rotational spin period of roughly 27 days is also consistent with such an age. Since HD 4747A and HD 4747B formed at the same time, both objects will have the same age. With a well constrained mass and age, HD 4747B is a good benchmark to test theoretical models of brown dwarfs.
Crepp et al. (2016), "The TRENDS High-Contrast Imaging Survey. VI. Discovery of a Mass, Age, and Metallicity Benchmark Brown Dwarf", arXiv:1604.00398 [astro-ph.SR]