Acamar

Acamar was the primary of the Acamar planetary system. There was an inhabited planet, Acamar III, in this system. This planet was the homeworld for the Acamarians, a warp capable humanoid species. 

Star Trek: Star Charts
Acamar (Theta Eridani): This was a binary star system. Acamar A was an A-class star with a magnitude of -1, which was a hundred times times brighter than Sol. Acamar B was an A-class star. The system was located in or near Federation space, in the Beta Quadrant. (page 56)

Startrek.com
Acamar (Theta Eridani) was located 50-60 light years from Sol.

Apocrypha
This star was mentioned as a destination in Pocket TOS: The Wounded Sky, and (with the "Theta Eridani" name) in Marvel TOS: Experiment in Vengeance! where the second planet in this system had a moon named Mycena. Neither of these depicted the existence of the native Acamarian civilization on the third planet.