On the voyage over, he fell in love with a fellow passenger, a German woman named Elizabeth Stöhr, known as Liesel. Two years later the couple had a daughter, Sonja, and two years after that they had twin boys. Jack, known as Jackie, was named for the boxer Jack Sharkey; Oskar, known as Ossi, for one of Liesel’s relatives.

Six months after the twins were born, Liesel, fed up with Josef’s womanizing and heavy drinking, ended the relationship. She scooped up Sonja and the more sensitive, fussy Oskar and returned to Germany. There her mother, who changed Oskar’s last name, had him baptized a Catholic and inherited parental duties when Liesel took a job as a nursemaid in Milan.

A strict Catholic and a harsh disciplinarian, Liesel’s mother made it clear to Oskar, when he asked her to explain what a Jew was, that he was never to repeat the word again. In school, when his principal asked him what sort of a name Yufe was — his sister had kept their last name — he replied that it was French, with an accent, pronounced yoo-FAY. Toward the end of the war, like most German children, he joined the Hitler Youth.

Jack, meanwhile, experienced his own brand of isolation, being Jewish (although secular) and white in predominantly black and Indian Trinidad, where the main religions were Christianity and Hinduism. At 7, he was told that he had a twin brother in Germany. Jack and Oskar both later said that during the war they had been haunted by the idea that they might one day meet on the battlefield, with one killing the other.

After the war, Liesel wrote to Josef asking for help and proposing a family reconciliation. But Josef, who had married a winner of the Miss Trinidad contest in the meantime, did not respond. Jack sent care packages of sugar and other island products.

At 13, Jack joined the Sea Scouts, an entry point for boys who wanted to join the British Navy. “I had to be very pro-British,” he told Professor Segal. “In my mind, being a Sea Scout lessened the importance of having a German mother.”

Instead, encouraged by an aunt in Venezuela who had survived the concentration camps, he went to Israel, where he worked on a kibbutz, served as a flagship sergeant in the Israeli Navy and married an American, Ona Hirsch, who urged him to make contact with his German family.