Star Born

From the first page of the Ace Double: "When Raf Kurbi's Terran spaceship burst into unexplored skies of the far planet Astra and was immediately made welcome by the natives of a once-mighty metropolis, Kurbi was unaware of three vital things: One was that Astra already harbored an Earth colony - descended from refugees from the world of the previous century. Two was that these men and women were facing the greatest danger of their existence from a new outburst of the inhuman fiends who had once tyrannized Astra. Three was that the natives who were buying Kurbi's science know-how were those very fiends - and their intentions were implacably deadly for all humans, whether Earth born of Star Born."