“In spite of my democracy / I’ve married the aristocracy / Like a true American girl.” Found in a song written in the 1890s, these lines refer to a new social type, the “dollar princess”—wealthy American heiresses obtaining old-world social status by marrying into the (mostly British) hereditary aristocracy. One man did more than anyone to fix the idea of the dollar princess in the popular imagination: the painter John Singer Sargent, who was never slow to spot an opening in the market. Sargent’s pictures of the Countess of Suffolk (also known as Daisy Leiter), Nancy Astor (formerly Nancy Witcher Langhorne), and the Duchess of Marlborough (born Consuelo Vanderbilt) are justly renowned examples of his high-style portraiture. The exhibition “Heiress: Sargent’s American Portraits,” a choice selection of eight oil paintings and 10 charcoals, is now on at Kenwood House, a 17th-century mansion perched on the edge of Hampstead Heath, in the northern suburbs of London. At one point Kenwood had its own dollar princess, Nancy Leeds, who lived there between 1917 and 1920. —Andrew Pulver