When Christie's auctioned off Edgar Degas' “Danseuses” for nearly $11 million in 2009, the catalog noted that the masterpiece was being sold as part of a restitution agreement with the “heirs of Ludwig and Margret Kainer,” German Jews whose vast art collection was seized by the Nazis in the years leading up to World War II.

But now a dozen relatives of the Kainers are stepping forward to object. Not only did they fail to benefit from that sale, they say they were never even told about it, or any other auctions of works once owned by the couple, including pieces by Monet and Renoir.