Most of the great Raphaels in existence are in London for the National Gallery’s survey, but one very special painting is traveling alone. The masterpiece La Fornarina, thought to be a portrait of the painter’s mistress, began a three-city tour last week in the Frick’s Oval Room. Its half-clothed subject wears the painter’s name on a ribbon around her arm and a coy expression that’s been likened to the Mona Lisa’s smile. While La Fornarina herself has never been conclusively identified (some scholars think she’s a baker’s daughter from Siena named Margherita Luti), her effect on Raphael is well documented. In his Lives of the Artists, Vasari claims that Raphael “could not give his mind to his work because of his infatuation for his mistress”; he even insinuates that this mystery woman may have caused Raphael’s untimely death (attributed to excessive sexual exertion)—all of which would explain why the painting, as striking as it looks, is technically unfinished. (At the Frick through January 30.)