For the second time in two months, a previously unknown portrait captioned "Emily Brontë" is to be auctioned, showing the Wuthering Heights author as a winsome but pensive young woman.

Painted in oils and with the subject gazing directly at the artist with clear brown eyes, the picture is less formal and possibly more flattering than the smaller, bonneted study that sold in December for £23,836, exceeding the reserve price of £10,000-£15,000.

Measuring 33 by 24cms (13 by 9.5ins), the painting has been reliably sourced to the mid-19th century and has a note of the subject probably made by the artist around the time of painting. But absolute attribution is unlikely, as has been the case with most supposed Brontë portraits apart from the famous study of the sisters painted in 1835 by their brother, Branwell.

The painting has been sent for auction by the Northamptonshire firm JP Humbert, which handled the "bonnet picture" sale. Jonathan Humbert said a private owner brought the portrait into the firm's office after reading about the previous sale. "One unknown portrait of Emily Brontë is lucky enough, but two in two months is quite remarkable," he said. "I am amazed that both have turned up on our doorstep."

Anything with a Brontë tag appears to sell well, although uncertainty about the authenticity of the latest picture has seen the reserve set at between £3,000 and £4,000. Last month the Haworth Parsonage museum, which has the world's greatest trove of Brontë relics, was outbid by a Paris museum for a miniature magazine made by Charlotte Brontë when she was 14.

The dainty handwritten manuscript was bought at Sotheby's by the Musée des Lettres et Manuscrits for £690,850, more than twice the reserve and a record for a literary work by any of the three sisters. The price of the bonnet painting was driven up on the same day by determined phone bidding to Northampton from the US.