I just discovered that Ancestry were providing free access to the 1940 US Federal Census until the end of today (but that's probably 7am British Summer Time).
I hadn't realised that the 1940 US census was available and I thought I would find where great uncle, Arthur Zentler, was living but he's not in it and neither is his daughter, Ruth, or his wife, Ida !
I commented on this to my brother and he told me that Ruth was studying chemistry at the Sorbonne in Paris, France when World War II broke out and she had to flee the country via Portugal, so perhaps her parents went over to France to help her leave and that's why they're not in the 1940 US Census.
So that's another story to research.
However, I did find the man that Ruth later married, so that's useful.