\defaultfontfeatures[XCharter]
  {
  Extension = .otf ,
  BoldFont = XCharter-Bold,
  ItalicFont = XCharter-Italic,
  BoldItalicFont = XCharter-BoldItalic,
  UprightFont = XCharter-Roman,
  SmallCapsFeatures =  {Letters=SmallCaps}
  }