I assume since they were for sale they are hardy enough.
Never a good assumption. most plants are grown in poly-tunnels, nice cozy warm places, away from the elements.
However lupins are very hardy - once hardened off - even in pots.
How big are the plants/pots.
I grow mine from seed and am just getting round to sowing a few that will grow well this year and flower next. they will be left out all next winter - in 3 ltr pots.
You are doing the right thing by not leaving them out at night to freeze, but when outside, keep them from getting too much (ice-cold) rain - the poor little root-lets dont like it.
I'd either bring in until March, putting them in to a cool bright, but not direct sun lit position, or as TG suggests, make them cloches, but still bring them in if its a hard long frost (if the soil is quite wet, this will kill the roots.).
Good luck.