But first, on July 4, Phillips is offering a picture that at least one dealer has waited long to rediscover. "I've fantasized about finding that picture for over 20 years," said Guy Stair Sainty, a bit glumly. "It's one of the most ravishing masterpieces by the artist. I just hope now that I'll be able to have a chance of buying it!"

The picture Sainty so ardently craves is The Return from the Ball (Le Retour du Bal) by Jean François de Troy (1679-1752), which Phillips estimates at £400,000-£600,000 pounds, or about $580,000-$870,000. He will have considerable competition.

Its companion, The Departure for the Ball, is in the J. Paul Getty Museum in Los Angeles, and its paintings curator, Scott Schaefer, has made no bones about wanting to reunite the pair. Unfortunately (for the Getty), this is also a picture that is sexy without being coy, a candle-lit interior rich and mysterious, in beautiful condition (the Getty's companion picture is somewhat rubbed) and apt to strongly appeal to people who normally dislike 18th-century French pictures.

One of the most appealing aspects about both pictures is their ambiguity. The Getty canvas depicts a group of cloaked partygoers clutching masks, as they hover about a lady at her toilette, whose maid is giving her powdered hair some last-minute finishing touches. A man and women whisper conspiratorially in the background shadows, while two men who appear to be the woman's husband and lover seem to be asking her to hurry up.

The Phillips canvas, the superior of the two, shows the same party back home. The Lady's smiling maid is removing her mistress' deep pink cloak, revealing her long neck and white shoulder. The Lady gazes slyly towards her lover, who crouches behind a chair and gestures towards a vacant room. In the background, the husband, happy to be home, gazes at the cozy hearth. In the background, our second couple sits alongside the fireplace, the woman playfully tapping the nose of her sleeping friend with her folded fan.

Much less well known than his contemporaries Watteau and Boucher, Jean François de Troy was in his day unusually successful and well-connected. His father was the court portraitist François de Troy, and having married a beautiful and wealthy aristocratic woman, Jean François was the envy of his colleagues.

Unfortunately, Jean Francois was sublimely arrogant and obnoxious in the extreme. He immodestly proclaimed that he knew of no greater painter than Veronese -- other than himself, further stating that he designed his compositions in his head without the need of preparatory drawings, and that once he set his ideas on canvas he never repainted or corrected them.

Like the 17th-century Italian painter Luca Giordano, de Troy was known as "Fa Presto" (paints quickly), and cranked out dozens of mythological and religious pictures with enormous speed. His contemporaries seethed at his effortless success, and while contemporary critics acknowledged his abilities, they were quick to fault the artist for his careless, excessive facility and his tendency to make all his doll-like female figures look alike (a fault he shares with Boucher). "Had de Troy added serious study to his natural talent and genius," said one, "he might have been remembered as one of the greatest French painters who ever lived."

To modern eyes, de Troy's bread-and-butter decorative paintings are pleasant if mostly unmemorable, no better and no worse than other painters of the day. It is only in the rare "tableaux de modes" (of which the Phillips canvas is a prime example) that he reveals his real genius. Indeed, these genre pictures (which number less than 20) are of such a consistently high level of inventiveness, finish and quality that it is difficult to imagine they are the works of the same painter.

Very few of these pictures are in America, the most notable being the Getty Departure for the Ball and the two sparkling canvases, The Garter and The Amorous Proposal, in the Wrightsman Collection, New York.

In such works, De Troy, the suave libertine, casts a bemused and indulgent eye on the world he knew well, as observer and participant. He may have been a bastard, but he sure had fun. "De Troy was a careless man, a heavy drinker and womanizer," says Sainty. "He only really put himself out for the most well-connected, well paying clients."

Both the Getty and Phillips pictures were painted for Louis XV's Secretary of State of Foreign affairs, Germain Louis de Chauvelin, who, following a fall from power engineered by his political rivals, never received them. It's unknown who wound up with the pictures, but they were engraved and frequently copied (usually in reverse) becoming among de Troy's most popular and best-known images. They are cited in Paris sales in 1749 and 1769, then vanish.

The Getty picture turned up in Shropshire, England, in 1948, and was bought from Wildenstein in 1984. The Phillips picture remained untraced until earlier this year. "It belongs to a family in Norfolk in East Anglia," says Phillips' Mark McDonald. "A local Phillips representative was sent photos of it. The clients had had it since the 1930s, but had no recollection or records of where the family acquired it. They were completely unaware of its value. It was kept under the stairs, as their mother didn't like it and was apparently was going to toss it out at one stage, which is quite a drastic thing to do, even for a picture you dislike."

Almost as drastic are the price discrepancies for works by De Troy. Prices for his works range from a few thousand to a few hundred thousand dollars. A Woman Holding a Peach sold for just $5,502 at Galerie Koller, Zurich, in March 1999, and a Virgin and Child sold in Paris last December for $22,811. At the Lagerfeld sale at Christie's last month, the dull Esther Fainting was bought in at $50,000 (est. $130,000-$160,000), while the excellent Pan and Syrinx made $160,000.