[XeTeX] fontspec and \emph

[Kenneth Reid Beesley]

If I use fontspec

\usepackage{fontspec}

and specify

\setromanfont[Mapping=tex-text]{Doulos SIL}

the problem is that Doulos SIL contains only roman glyphs
and \emph{foo} does not come out in italic.  Is there an
easy way with fontspec to use Doulos SIL in this way but
have it slip into another font, e.g. Charis SIL, for rendering
text inside \emph{}?

Thanks,

Ken