I answer now my own question (sorry!): just use: t means
object t = Type.Missing;
//XlFileFormat.xlExcel8 suits to .xls format
worksheet.SaveAs("Test.xls", XlFileFormat.xlExcel8, t, t, t, t, t, t, t, t);
and saving as office2007:
worksheet.SaveAs("Test.xls", t, t, t, t, t, t, t, t, t);