with the US Strategic Bombing Survey, studying the effects of Allied bombing on German morale, an experience that affected his postwar work as his visit to Spain had affected him earlier. On his return, he settled in Manhattan, working as a freelance writer, a lecturer at The New School for Social Research, and a visiting professor at Bennington, Smith, and other American colleges. In 1946, he became a naturalised citizen of the US. In 1948, Auden began spending his summers in Europe, together with Chester Kallman, first in Ischia, Italy, where he rented a house. Then, starting in 1958, he began spending his summers in Kirchstetten, Austria, where he bought a farmhouse from the prize money of the Premio Feltrinelli awarded to him in 1957. He said that he shed tears of joy at owning a home for the first time. His later poetry, mostly written in Austria, includes his sequence "Thanksgiving for a Habitat" about his Kirchstetten home. Auden's letters and papers sent to his friend the translator Stella Musulin (1915-1996), available online, provide insights into his Austrian years. In 1956–61, Auden was Professor of Poetry at Oxford University where he was required to give three lectures each year. This fairly light workload allowed him to continue to spend winter in New York, where he lived at 77 St. Mark's Place in Manhattan's East Village, and to spend summer in Europe, spending only three weeks each year lecturing in Oxford. He earned his income mostly from readings and lecture tours, and by writing for The New Yorker, The New York Review of Books, and other magazines. In 1963, Kallman left the apartment he shared in New York with Auden, and lived during the winter in Athens while continuing to spend his summers with Auden in Austria. In February 1972, Auden moved his winter home from New York to Oxford, where his old college, Christ Church, offered him a cottage, while he continued to spend summers in Austria with Kallman. He spent only one winter in Oxford before his death in 1973. Auden died at 66 of heart failure at the Altenburgerhof Hotel in Vienna overnight on 28–29 September 1973, a few hours after giving a reading of his poems at the Austrian Society for Literature. He was intending to return to Oxford the following day. He was buried on 4 October in Kirchstetten, and in London a memorial stone