The eccentric German philanthropist Gustav Rau spent part of the last century running an automotive parts empire he inherited from his father. Then, at the age of 40, he enrolled in medical school, became a physician, and moved to Africa, where he built a hospital in the Belgian Congo. The one constant in Rau’s extraordinary life was a love of Old Master paintings. Part of his collection, with works ranging from the late Gothic to the mid-18th century, will be sold tomorrow evening at Sotheby’s London headquarters.

Both major auction houses will hold London sales of significant Old Master works this week that fall directly in the wake of record-breaking sales earlier this season.

“Rau bought what he truly loved,” the co-chairman of the Old Master Paintings department at Sotheby’s, Alexander Bell, said. “It’s probably not how most buyers of Contemporary art approach collecting today.”

Indeed, the Old Master school has yet to command the stratospheric auction prices seen at recent Contemporary and Impressionist sales. But with several re-discovered paintings and drawings in the mix, the Old Master auctions could be a frenzy.

The marquee work up for auction at Sotheby’s is J.M.W. Turner’s “Pope’s Villa at Twickenham” (estimated at between $9.9 million and $13.85 million). Twinkenham, in southwest London, was where Turner watched the home of the poet Alexander Pope fall into disrepair. The large oil-on-canvas painting has been in the collection of the Morrison family since 1827 and until recently hung in their country house in Gloucestershire, Sudeley Castle.

Another notable work in the Sotheby’s sale falls at the other end of the size spectrum. At just 10 by 14 ¾ inches, Jan Brueghel the Elder’s painting is in some ways smaller than its name: “The Edge of a Village with Figures Dancing on the Bank of a River and a Fish-Seller and a Self-Portrait of the Artist in the Foreground” (estimate of

between $4.95 million and $6.93 million).

“The fine, polished surface and meticulous detail certainly makes this one of the greatest things he ever painted,” Mr. Bell said.

Then there are the lots that once belonged to Rau, who died in 2002 at the age of 80. “The strength of this man’s collection rests with the fact that he bought the best picture of the second and third best painter in any particular school,” Mr. Bell said. “It’s a strategy that worked; he didn’t try to buy the lesser works of better artists.”

Rau was known for flying a small airplane to Europe — a “rattletrap,” according to Mr. Bell — from his village in the Congo, and, still wearing his desert boots and khaki fatigues, striding into the finest galleries and art sales intent on making purchases. Initially he took the advice of buyers, but increasingly began buying what moved him.

Rau’s plan was to build a gallery in Marseilles, France, to display the 1,000 objects that he amassed over a 30-year period. Although he never completed the gallery, he staged an exhibition of many of the paintings at the Palais de Luxembourg shortly before he died.

Several charitable organizations are contesting his estate; Rau had no heirs. The estate’s executor has permission to sell enough paintings to cover the cost of running the estate and the hospital in Africa, according to Mr. Bell.

The German philanthropist’s tastes ranged all the way to mid-18th-century works such as Jean-Marc Nattier’s “Portrait of the Comtesse D’Andlau” (estimated at between $297,000 and $396,000). A tempura-on-panel triptych by Taddeo di Bartolo has an estimate of between $595,000 and $990,000. But Mr. Bell said he expects its well-preserved condition to command a much higher price.

Di Bartolo used gold ground throughout the work, which features the Madonna and Christ Child in the central panel, flanked by St. John the Baptist and St. Jerome. It dates from around 1395 and stands as an example of the artist’s late Gothic style.

Christie’s holds Old Master sales this evening and tomorrow, the latter of which will include paintings belonging to the small nation of Lichtenstein’s royal family.

This evening the big surprise at Christie’s is, well, the appearance of the Flemish painter Jean-Antoine Watteau’s “La Surprise.” The painting, long assumed destroyed, turned up last year in the corner of a drawing room in a British country estate during a valuation of its furnishings.

Christie’s estimates the painting could sell for between $5.93 million and $9.88 million. “We are extremely excited to have rediscovered Watteau’s ‘La Surprise’,” the director and head of Christie’s Old Master Pictures in London, Paul Raison, said.

A Swiss family experienced a similar surprise when a cursory inventory of its home turned up three drawings from the private albums of Francisco Jose de Goya y Lucientes. One member of the family had a hunch that the sketches looked better than everything else in the collection. “He turned out to be right – they’re Goyas,” the director and international head of Old Master Drawings at Christie’s, Benjamin Peronnet, said.

None of the Goya drawings up for auction in London this evening were ever mounted and framed. The artist numbered them all, allowing Christie’s to connect them to series of drawings on similar topics.

For instance, one of the drawings, “The Constable Lapinos Stitched Inside a Dead Horse,” is numbered 85 in a series of sketches in Goya’s Album F. Drawing no. 86 hangs in the Metropolitan Museum of Art and shows the death of Lapinos after a tortured night inside the rotting animal. Christie’s estimates the work could sell for between $1.19 million and $1.58 million.

“These drawings show how Goya is a bridge from the Old Masters to Contemporary Art,” Mr. Peronnet said.