We appear to have gained a cat, at least for the night. The little tabby we've named Oliver was shivering on the doorstep for about an hour. I called the emergency vet, and they assured me that as long as I kept him quarantined in the bathroom, Ruby would be safe from any disease.
There's a freeze warning for tonight, and I couldn't leave him outside. So I'll have to take him to my vet on Friday and have him tested for communicable diseases and checked for a chip. If he's clear, and Ruby gets along with him, we'll have a new kitty.
He chowed down a big bowl of food in about a minute, and gulped down some water, so I think maybe he has been abandoned. He was too hungry, too thirsty, and too frozey to have a safe place to go.
So he's warm, fed, and safe in the bathroom, having brief conversations over the baby gate at Ruby, and I think we did the right thing. I think. I hope. I couldn't bear the thought of him outside all night. Too cold. And he's awfully friendly toward us. He purrs like a motor boat when you talk to him.