Yadera II was the second planet in an unspecified system located in the Gamma Quadrant.
Yadera II was named after Yadera Prime, homeworld of the Yaderans. It became the refugee Rurigan's new home after the Dominion invaded his homeworld of Yadera Prime in 2340. With the help of a hologenerator, he created sentient holographic lifeforms on the planet. (DS9: "Shadowplay")
Background information Edit
The planet's name is not from the episode. Rather, it is from StarTrek.com and the Star Trek Encyclopedia (4th ed., vol. 2, p. 504).
According to the script, the pronunciation for Yadera was "yah-DER-ah". 
According to Star Trek: Star Charts (p. 75), Yadera II orbited a K-class star. Yadera II was a destination in the Gamma Quadrant Bio-Survey (2369-75).
According to the Star Trek Encyclopedia (4th ed., vol. 2, p. 504), the matte painting of the Yadera II landscape was re-used later for the planet Ronara Prime in "Preemptive Strike".