Under Heaven's Bridge — Michael Bishop

A multinational expedition has landed on the planet Onogoro, a cold and dour world circling one star of a binary pair. Their objective is to investigate a strange alien race, known to the human visitors as the Kybers. These aliens, dwelling in a great network of ruined palaces, are partly biological creatures and partly machines, with the ability to switch themselves off at will. Expedition scientists discover that the Kyber's sun is soon to blaze up in a nova, yet the Kybers are not alarmed.