[XeTeX] Changing font for equations

[Easter Sunshine]

Hello,

Is there a way to override the font used to typeset
equations? My mainfont was set to Minion Pro but some
of the characters are like $\circ$ and $\times$ are
not showing up. I'd like to revert to Computer Modern.

I've tried \setmathrm{Myriad Pro} and
\setmathsf{Myriad Pro} as a test but the equations
were still typeset in Minion Pro.

I've also tried loading packages euler and mathpazo
before fontspec but the equations were still typeset
in Minion Pro.

I've also tried issuing
\fontspec[ExternalLocation]{cmr10} $\circ$ but the
equation was still typeset in Minion Pro.

I've also tried using \setromanfont instead of
\setmainfont  but equations are still typeset in
Minion Pro.

I am using XeLaTeX shipped with TeXLive 2007 so I
don't have package unicode-math or fontspec's no-math
option. I'm afraid of upgrading because I cannot
afford to break my TeXLive installation when my report
is due in 15 hours.

Thanks for any help.
-Easter Sunshine


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