They used to be 27.2, but I've a feeling the XLMs went to 31.6. Front mechs changed from 31.8 to 34.9 around the same time.
How old an Oreas? The changes to the XLM happened when they went from 3al 2.5v to 6al 4v titanium, if the Oreas is still 3/2.5 it may be 27.2 and 31.8.