HD 136118 is a star in the Serpens Caput section of the Serpens constellation. The star is too dim to be readily visible to the naked eye, having an apparent visual magnitude of 6.93.^[2] It is located at a distance of 165 light years from the Sun based on parallax,^[1] and is drifting closer with a radial velocity of −3 km/s.^[2]

This object is an F-type main-sequence star with a stellar classification of F7V.^[4] The absolute visual magnitude of this star suggests that it has begun to evolve away from the main sequence.^[3] The abundances of the stellar atmosphere are similar to the Sun, and it has only a modest level of chromospheric activity.^[9] HD 136118 has 84% more mass compared to the Sun,^[6] and is 70%^[7] larger in radius. The star is an estimated 3.5^[2] billion years old and is spinning with a projected rotational velocity of 8.5 km/s.^[5]

The astronomer Debra Fischer discovered a substellar companion, originally thought to be a very massive exoplanet, which was announced on February 7, 2002.^[3] Designated HD 136118 b, it is orbiting the host star with a period of 3.25 years.^[9] This object has a minimum mass of 11.9 M_J.^[3] On November 25, 2009, its inclination was calculated to be 163.1° and its true mass 42 M_J, classifying it as a brown dwarf.^[9] Later studies in 2022 and 2023 found true masses of about 13-16 M_J, closer to the minimum mass, but still classifying the companion as a brown dwarf by most definitions.^[10][11]

Due to its high mass the object is likely to be very hot and possibly glowing faintly. The orbit of the object has a semimajor axis of 1.45 astronomical units from the parent star, taking 1,188 days (3.25 years) to complete one eccentric orbit.

• List of brown dwarfs