Got some mongrel cats living in one of the barns all year. They get fed around the neighbourhood, but seem to prefer to hang in the lowest level carved out the hill under the barn, where they make themselves nice little nests at a respectable distance from the chickens. I know they've been out there in subzero (F) weather, seem fine. They definitely have undercoats.
A few years ago i walked up a hill on a cloudless night to get to cell phone reception, was well below zero (F) with the stars hanging like diamonds and corn stalks breaking like gunshots under my boots. One of the cats accompanied me all the way up the hill and back, a good hour in very cold weather. Pranced about, ran after real and imagined prey, had a good old time.
I daresay the cat was warmer than i was.
sidd