According to StarTrek.com, Ekos was located in the M34 Alpha system; however, according to the Star Trek Encyclopedia (4th ed., vol. 2, p. 513), the planet was located in the M43 Alpha system. Furthermore, it is stated in the Star Trek Encyclopedia that Zeon was a M-class planet.
According to the Star Trek Maps (p. 27) and The Worlds of the Federation (p. 122), the name of the system was M43-Alpha. It was a trinary system, Zeon being the fifth planet. According to Star Trek Maps, the total population of the two planets was 16.1 billion.