Frost on the way?

We have a frost warning this evening, so I covered my tomatoes and peppers with my portable cold-frames.  I just harvested my first cucumber today and my first two tomatoes in the past two days, so  I hope I can extend their season a bit.  I only have maybe three more cukes on the vine, and four more tomatoes, but I've waited so long for them I'd hate to lose them now.