[XeTeX] XeTeX and ligatures for dashes

[heliotropion]

Greetings, list!

I seem to be having a problem (described here:
http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&item_id=xetex_faq#ligs):
dashes are not replaced by the appropriate ligatures in XeTeX. I am
using the OpenType version of the Computer Modern
typeface ("CMU Serif"), introduced into the preamble by
    \setmainfont[Mapping=TeX-text]{CMU Serif}
I have also tried inserting
    \defaultfontfeatures{Mapping=tex-text}
yet the ligatures won't show.

Any clues?

Thanks,
Al.
-- 
The jewel that never leaves the mine is never polished. -- Saadi