Constanthus

Constanthus is an outer colony world within the Romulan Star Empire and was ruled with an iron fist by its governor. One of the cities of the planet is Auranthus.

It, like Romulus, had a Unification movement which urged for reunification with the Romulan cousin species, the Vulcan's. Spock headed the movement and arrived Constanthus but was betrayed by a spy that alerted Romulan security forces which quickly captured those local unificationists and those that journeyed from the Romulan homeworld.

(TNG novel: Crossover)