In his paper, he describes the expected lightcurve signatures expected from passing objects of various shapes across a sun. The challenge is to use a shape that provides an unambiguous signature.
The ring was checkered. There were regular rectangular shadows along its blue back.
"Can you give us a better picture?"
"We can expand it," said the contralto voice. The G2 star jerked forward, then shot blazing off to the right, so that Louis was looking down on the lighted inner surface of the ring. Blurred as it was, Louis could only guess that the brighter, whiter areas might be cloud, that regions of faintly deeper blue might be land where lighter blue was sea.
But the shadowed areas were quite visible. The ring seemed to be laid out in rectangles: a long strip of glowing baby blue followed by a shorter strip of deep, navy blue, followed by another long strip of light blue. Dots and dashes.
"Something's causing those shadows," he said. "Something in orbit?"
"Yes, just that. Twenty rectangular shapes orbit in a Kemplerer rosette much nearer the primary. We do not know their purpose."
"You wouldn't. It's been too long since you had a sun. These orbiting rectangles must be there to separate night from day. Otherwise it would always be high noon on the ring."
Here's a view of the shadow squares, which are used to provide periodic darkness on the surface of the ring.
The ringworld and its shadow squares would also provide a light curve that would be recognized as artificial from a great distance.
Read more about the story at Look out for giant triangles in space; see more Ringworld art. Go for the hard stuff at Transit Lightcurve Signatures of Artificial Objects (pdf). Speaking of blocking out the sunlight, see the article on sf writer Gregory Benford's suggestion of reducing global warming by blocking sunlight.
(This Science Fiction in the News story used with permission from Technovelgy.com - where science meets fiction.)