[XeTeX] titlepage is not centered in the horizontal direction

[Stefan Pohl]

Hi list-members,

The option plain causes the headlines, footlines, and sidebars to be
suppressed. This is useful for creating
single frames with different head- and footlines or for creating frames
showing big pictures that
completely fill the frame.

But what do I have to do on the titlepage where are no head- and footlines.
With the plain option the 
head- and footlines are only invisible and so the titlepage is not centered
in the horizontal direction. 
 
\begin{centering} and \end{centering} does not work.

\begin{frame}[plain]
\begin{centering}%
 \titlepage
\end{centering}%
\end{frame}

Thank you for your help. Stefan.