overpic package

used for put some latex characters to xfig output figure;

\usepackage{overpic}
\includegraphics[width ...
\put(-20,40){$X_p$}

or

\begin{overpic}[scale=.5]{abc.eps}
\put(-20,45){ $X_{post}$}
\end{overpic}

don't use both of them, otherwise you will get 2 copies