  Here we present an in-depth analysis of the periodicity of the continua and
broad emission lines of a quasar E1821+643. We applied non-parametric composite
models, the linear sum of stationary and non-stationary Gaussian processes, and
quantified contribution of their periodic parts. We found important qualitative
differences among the three periodic signals. Periods of $\sim 2200$ and $\sim
4500$ days appear in both continua 5100 \AA \, and 4200 \AA \,, as well as in
the H$\gamma$ emission line. Their integer ratio is nearly harmonic $\sim
\frac{1}{2}$, suggesting the same physical origin. We discuss the nature of
these periods, proposing that the system of two objects in dynamically
interaction can be origin of two largest periods.
