  We searched the orbital period changes in 182 EA-type (including the 101
Algol systems used by \cite{hal89}), 43 EB-type and 53 EW-type binaries with
known both the mass ratio and the spectral type of their secondary components.
We reproduced and improved the same diagram as Hall's (1989) according to the
new collected data. Our plots do not support the conclusion derived by
\cite{hal89} that all cases of cyclical period changes are restricted to
binaries having the secondary component with spectral types later than F5. The
presence of period changes also among stars with secondary component of early
type indicates that the magnetic activity is one cause, but not the only one,
for the period variation. It is discovered that cyclic period changes, likely
due to the presence of a third body are more frequent in EW-type binaries among
close binaries. Therefore, the most plausible explanation of the cyclical
period changes is the LTTE via the presence of a third body. By using the
century-long historical record of the times of light minimum, we analyzed the
cyclical period change in the Algol binary WW Dra. It is found that the orbital
period of the binary shows a $\sim112.2 \textbf{\textrm{yr}}$ cyclic variation
with an amplitude of $\sim0.1977\textbf{\textrm{days}}$. The cyclic oscillation
can be attributed to the LTTE via a third body with a mass no less than $6.43
M_{\odot}$. However, no spectral lines of the third body were discovered
indicating that it may be a candidate black hole. The third body is orbiting
the binary at a distance shorter than 14.4 AU and it may play an important role
in the evolution of this system.
