From tudor_chic's photostrem on flickr. The posting has this caption: "Diana Kirke was the rare woman who transitioned from mistress to wife: she married her lover, the Earl of Oxford, in 1673. In this painting, she has a breast exposed - possibly alluding to Venus (whose symbol was a rose, which Diana holds in her hand). She's wearing a popular Stuart-era color in her loose gown: amber orange."
This is her brief article from thepeerage.com, "Diana Kirke married Aubrey de Vere, 20th Earl of Oxford, son of Robert de Vere, 19th Earl of Oxford and Beatrix van Hemmend, on 12 April 1673. Her married name became de Vere."
The Duchess of Oxford wears a standard hurluberlu coiffure with a flat panel across the top of her head (eyes up, please) and a loose gown in this 1665 Lely portrait.
Keywords: 1665, Lely, Diana Kirke, Countess, British, long hurluberlu coiffure, décolletage, chemise, modesty piece, elbow length sleeves, over-dress, earrings