A painting by Austrian artist Gustav Klimt believed to have been lost for a century is to be auctioned in Vienna. However, there are several mysteries surrounding Bildnis Fräulein Lieser (Portrait of Fräulein Lieser), the unfinished work which Klimt began in 1917, a year before he died. There is some debate about who the woman depicted is and what happened to the painting during the Nazi era, the BBC reports . Experts believe it may be one of the daughters of either Adolf or Justus Lieser, brothers from a wealthy Jewish industrialist family. Art historians Thomas Natter and Alfred Weidinger say the painting is of Margarethe Constance Lieser, the daughter of Adolf Lieser. But Vienna auction house im Kinsky, which is selling the artwork, suggests it could also depict one of Justus Lieser's two daughters or his wife Henriette. The painting's history after 1925 is "unknown", according to the auction house. "What is certain is that it was acquired by a legal predecessor of the consignor in the 1960s and then passed to the present owner through three consecutive inheritances," it said. The current owners have not been publicly identified. The painting is being sold on behalf of them and the legal successors of Adolf and Henriette Lieser, under the Washington Principles – an international agreement to return Nazi-looted art to the descendants of those from whom it was taken. It is estimated to be worth up to €50m (£44m), although it could sell for more. The auction is due to take place later on Wednesday. Read More: Egg donated by poor elderly woman to mosque sells for €2,500 at auction (in Portuguese)