- You may also be looking for the planet Deneb IV.
Deneb Kaitos IV (commonly known as simply Deneb IV) is a Class M planet located in the Deneb Kaitos system. (ST reference: Star Charts) It was the homeworld of the Deirr. (TOS - Rihannsu novel: My Enemy, My Ally)
In the 2250s, James T. Kirk and Gary Mitchell visited the planet, and Mitchell became involved with a woman. For years afterward, Kirk remarked that he had been worried about Mitchell since their visit to Deneb. (TOS episode & novelization: Where No Man Has Gone Before)