To keep them warm, I would recommend using shelters they can go into during the cold winter nights. You have several choices, depending on your budget. If you look towards the bottom of my HubPage, you'll see "Works Cited." These articles have some great, free resources for you to read up on & then choose the best solution for you and the cats. These cited articles are only a few of the many free articles you can read. They will help you gain a whole wealth of knowledge about how to specifically keep your outside cats warm and healthy. There are many options, with various price ranges. I've no doubt you'll choose the right solution, at the right price range for you, as well as the cats.