Gradimir & Katharina arrived aboard the USNS General S.D.Sturgis which sailed out of Bremerhaven on 3 February 1952 and arrived in New York on 14 February 1952. He was aged 40, and she was 27. They were travelling on Yugoslavian passports. They gave their address in the US as 214 East 21st Street, New York.Perhaps you might find out more info by seeing who else lived at this address in New York in 1952 by contacting the NY Public Library and having them check their city directories of that time.
Katharina and Therese must have made a trip home because I find them arriving back in New York aboard the SS Ryndam which sailed out of Rotterdam on November 16, 1956 and arriving in New York on November 27, 1956. Katharina is noted as "stateless" while Therese is noted as a US citizen. Katharina's passport # was A8239574 and Therese's was 545242 (issued in Gary, Indiana)