Cω is an extended CeeSharp variant supporting SemanticSubtyping with XML and relational datatypes, and concurrency abstractions based on the JoinCalculus - the mutant offspring of MicrosoftXen and PolyphonicCeeSharp. The language name is capital C, lowercase Greek omega.
There's a downloadable preview (requires VisualStudio .Net 2003), available at http://research.microsoft.com/Comega/, and some documentation, at http://research.microsoft.com/Comega/doc/comega_startpage.htm
Originally at MicrosoftXen, about the typing features:
I believe it is a research lab creation to demonstrate OoAndXml can co-exist, and that OoVsRelational is a nonissue [ObjectRelationalImpedanceMismatchDoesNotExist].
See Programming with Rectangles, Triangles, and Circles at http://lambda-the-ultimate.org/classic/message9178.html
See also the discussion of the JoinCalculus at ActorsModel.