A work previously belonging to Sargent's friends Wilfred & Jane de Glehn, and unknown to the Sargent catalogue project, is set to go on sale at Christie's in November. It's attribution to Sargent has been confirmed by Elaine Kilmurray of the Sargent catalogue project.
From the Christie's Lot Essay:
The present study is undated and the model unidentified. A young girl is represented in profile, seated cross-legged beside a stream, reading a book. Her dress is difficult to pin down, reading more as picturesque costume than as a contemporary garment. There is no horizon to provide context, the picture plane is tipped up so that the water provides the reflective background, and there is a sense created of an enclosed natural space. This is consistent with Sargent’s figure studies of the period, which are usually placed against an aqueous, rather than an aerial, backdrop. The composition exhibits a technical affinity with a small study of Sargent’s sister, Violet, standing on a river bank with a fishing rod in her hand, a preliminary study for Lady Fishing - Mrs Ormond (Tate Britain). The similarity of the broad treatment of the reflections in the water is particularly close.
Girl Seated by a Stream is an important rediscovery and was in the collection of the artist’s friends Wilfrid de Glehn (1870-1951) and his American wife, Jane, née Emmet (1873-1961), who became significant figures in Sargent’s life after their marriage in 1904. They were both artists and, for the following decade, they accompanied Sargent on his painting expeditions to the Italian Alps, Florence, Rome, Frascati, Corfu, Granada and Lake Garda. Sargent and de Glehn often painted the same motif, seated side by side, and the couple were models in a number of Sargent’s pictures. Jane was known for her portrait drawings in sanguine chalk and for her landscapes in oil. Wilfrid, who had studied with Jean-Paul Laurens in Paris and with G.P. Jacomb-Hood in London, painted portraits and subject pictures in a distinctly Sargentesque style.
You can read more about it in the Christie's catalogue here.