  O-C diagram is a useful technique to analyse the period changes of a pulsator
by using the maximum (or minimum) value points which have been obtained from
the historical data. But if an object is a double-mode or multi-mode pulsator,
the extreme value points are the results of all the modes other than just the
fundamental mode. We discussed these situations and give out some criteria to
judge whether the O-C diagram can be used in these situations.
