[XeTeX] Oldstyle figures in seperate file

[Wolfgang Keller]

Hello,

if the oldstyle figures are provided (together with the small caps) in
a seperate font file (this is a truetype font), how can I tell XeTeX to
use these for all (roman) numbers?

Or do I have to format all my numbers explicitly as small caps to get
the oldstyle figures?

TIA,

Sincerely,

Wolfgang