  Periodicity search in gamma-ray data is usually difficult because of the
small number of detected photons. A periodicity in the timing signal at other
energy bands from the counterpart to the gamma-ray source may help to establish
the periodicity in the gamma-ray emission and strengthen the identification of
the source in different energy bands. It may, however, still be difficult to
find the period directly from X-ray data because of limited exposure. We
developed a procedure, by cross-checking two X-ray data sets, to find candidate
periods for X-ray sources which are possible counterparts to gamma-ray pulsar
candidates. Here we report the results of this method obtained with all the
currently available X-ray data of 8 X-ray sources. Some tempting periodicity
features were found. Those candidate periods can serve as the target periods
for future search when new data become available so that a blind search with a
huge number of trials can be avoided.
