Powerful portraits of seated men by Lucien Freud are offered at both Christie's and Sotheby's London sales of contemporary art next week. At Christie's on February 8, one of Freud's best and most powerful portraits, Man in a String Chair, 1988-9, is also a magnificent composition. The full-length oil is of the gambling czar known as the Gentleman Bookmaker, Victor Chandler ( 3m.- 4m.). The oil was not a commission; Chandler, a good friend, consented to sit for Freud four times a week. A quite superb and recognizable self-portrait by the late Francis Bacon, 1969, from the estate of Valerie Beston, the key Marlborough Gallery employee who died last year, has a top estimate of 1.8m. There is no public disclosure of the estimates for another Bacon on offer, a scruffy study for one of his versions of the riveting portrait of Pope Innocent X by Velazquez (for my money the greatest portrait ever painted). Also from Beston's collection are several gestural oils by Frank Auerbach. At the Sotheby's sale next Thursday is a wide range of works by many of the key figures working from the 1950s onwards - Freud, Bacon, Lichtenstein, Richter, Klein and de Sta l, among others. The Portrait of Bruce Bernard 1996, by Lucien Freud, is a gloomier and more static work than the Christie's lot. Bernard was also a close friend of Freud's and the present portrait (estimated at 2m.- 3m.) is mooted as one of the artist's most probing representations. I prefer the Chandler. The Sotheby's sale also includes a number of important sculptures, among them, Eduardo Chillida's Silent Music II, recently sold by the Metropolitan Museum ( 1m.- 1.5m.).The evening sale carries the highest estimate for any winter season sale of contemporary art at Sotheby's London. It will be followed by a day sale on Friday the 10th.