bad interaction: beamer, amsmath, preview-latex

[Giacomo Boffi <giacomo.boffi@polimi.it>]

_[xposted, followup set to gnu.emacs.help]_

i have two files, one (yb.tex) that uses the beamer class and another
(nb.tex)  that uses the article class

when i use preview-latex to generate inline pictures of my equation,
in the first case it does not work, while in the second case it works
as usual

environment: auctex and preview versions are "11.86", XEmacs 21.5
(beta29) "garbanzo" 39d74978fd32 [Lucid] (i686-pc-linux, Mule)

thank you in advance for your help,
                                                                gb
,----[ yb.tex ]
| \documentclass{beamer}
| \usepackage{amsmath}
| \begin{document}
| \begin{frame}
|   \frametitle{pipp}
|   \begin{align*}
|     a & = b \\
|     c & = d
|   \end{align*}
| \end{frame}
| \end{document}
`----

,----[ nb.tex ]
| \documentclass{article}
| \usepackage{amsmath}
| \begin{document}
| \begin{align*}
|   a & = b \\
|   c & = d
| \end{align*}
| \end{document}
`----
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