Excerpt from an SF novel in progress:
Keelo chose to enter the system from a direction which put them in a position for a dramatic view. They came out of drive with the star ahead of them - which was, of course, normal. Beyond it was a thin, shining ring, fully illuminated by its star.
Take a narrow tube. Rotate it around its long axis to simulate gravity on the inner surface, after the fashion of an O'Neill colony. Give it a slight curve and extend it until the ends join to make a vast circle. Partitioning this into world-sized chunks so one hole doesn't leak all the air. The radius of curvature of the ring as a whole would be so great with respect to the tube's diameter that the flexing from rotation would be trivial, especially with the sorts of materials the Pertarn had. Orient the ring so that the large axis of rotation points through the parent star. Allow the portions above and below the ecliptic to curve away from the star through orbital dynamics. This last to give seasons as the ring rotated, two "years" per full turn.
Several such habitats were known, most of them created by the Pertarn. This was the most nearly intact of them, and also the most nearly intact of all known surviving large Pertarn constructs, which involved far more classes of objects than just habitats. This object had nearly a quarter of the sections open to space or otherwise no longer habitable. Even so, it had vastly more viable land surface remaining than any habitable planet. Various cultures had made use of - and even repairs to - the structure since the fall of the Pertarn. Something which would have given those xenophobic former masters of the Local Great Cluster fits. Currently, the habitat contained a mix of ecosystems, with little of the original life remaining. Some of that change was due to simple evolution. Which would also have upset the inflexible Pertarn.
The primary was an elderly K3 star, unremarkable except for the habitat and associated facilities.
"Just think," said Gail, wistfully, "only a little over a billion years before it starts going red giant. I wonder if anyone will bother to save the habitat."