[XeTeX] mapping and Latin Modern font

[Alex Hamann]

I am using a utf-8 encoded tex-file, process it with xelatex using  
xdvipdfmx and want to have it typset with latin modern fonts.
calling
   \usepackage{xunicode, fontspec}
gives me everything I want except for the correct mapping of  
ligatures e.g. "--" and "---"
 From the fontspec manual I could only gather how to get the correct  
mapping when using the setromanfont-option.
How can I achive both, i.e. latin modern fonts (the defautl font when  
not calling for any specific font, right?) but also the correct mapping?

Cheers,

Alex