They could stay out to about 60-55*F, but that's night-time temp. You couldn't keep them outside without some heat to bask with and digest their food. I have had mine outside in the warmer months, but not at all during the few weeks of most cold, it's way too uncomfortable for them.
I'd also bring them in so they can get warm enough. We've been lucky enough that it's been a pretty warm winter so far, but when it gets cold it gets too cold for them.