Question
what is the lowets temperature african violets can support in an outside terrace?

Answer
Judith,
African violets are one of the few plants which must have warmer temperatures to not only thrive, but to LIVE.  Most reference books will state a minimum temperature of 65-70, and never to drop below 60.  The violet may not die at 60 degrees, but neither will it bloom.  However, a violet will perish at temperatures below 60, and closer to 50.

Violets are extremely sensitive to temperature changes, either extremes.  Also, not to be neglected is humidity.  Violets cannot tolerate a low humidity.  I hope this answers your question.