How much cold can a horse take? please help urgent?

some of my horses are normally stabled at night. i wont be able to stable them for the next week and i wont be there to take there rugs on and off. they will be out in the paddock. its winter here and the lowest it gets a night is about -5 degrees and it frosts but the hoteset it gets up to during the day is about 15 degrees. it is also spose to be raining and windy for the time. the only rugs i have are really heavy weather beater rugs so should i leave their rugs on for the whole time ( will they get to hot) or should i leave them off ( will they get to cold) ??????????????