Apostolos,
preface = \textPi \textrho\acctonos \textomicron\textlambda
\textomicron\textgamma
XeLaTeX is Unicode aware and can handle Unicode strings. Therefore, I fail to
see
why you are doing things this way. The LGR font encoding is an ancient hack that
has no usage anymore.
Of course, in Unicode engines the default captions section
apply, not the captions.licr subsection.
Javier
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