A rare opportunity to see, and buy, a series of paintings by Frank Auerbach occurs this month when eight portraits of the art historian Ruth Bromberg will be offered for sale at Sotheby’s. Revealed for the first time, though not for sale, is the correspondence between this most private of artists and one of his sitters.

Auerbach has long been the sleeping giant of figurative painting in the postwar era. Grouped with his fellow London painters, Francis Bacon and Lucian Freud, he has not attracted the same prices, but he has commanded comparable respect. As a portrait painter, he is often compared to Giacometti for the intensity with which he paints and repaints a small but select number of sitters, scraping away his earlier efforts as he struggles to express the truth.

Ruth Bromberg fled Nazi-occupied Europe in the Forties and landed in New York, where she met and married Joseph, a successful furrier. At about the same time, Auerbach was despatched by his parents from Berlin to England for safety, aged eight, never to see them again.

Apparently self-taught as an art historian, she produced a complete catalogue of the etchings of Canaletto in 1974. In the early Eighties, she and Joseph suffered the loss of their only child, Michael, from a brain tumour, aged 28. Art thereafter became something of a refuge. Having worked for the art dealers Colnaghi’s, she developed an interest in the modern British painter Walter Richard Sickert, after her husband gave her a Sickert print for her birthday. Discovering there was no reference work on Sickert’s prints, she embarked on producing a catalogue of them, which came out 10 years later, in 2000.

Sickert was also an inspiration to Auerbach, whose roots were similarly in Camden Town. Ruth and Joseph were already the proud owners of two Auerbach paintings when they were introduced to him by David Landau, the founder of Print Quarterly, who was sitting for Auerbach during the Eighties. In 1991, Joseph wrote to Auerbach to ask him to paint Ruth’s portrait and, although the artist rarely accepts commissions, he agreed.