observed times in a similar way that the O-C diagram described by @2005ASPC..335....3S does it. As eclipse timings we choose the fainter points in the colored data strings shown in Fig.3. We find a period rate of change of dP$_{\rm o}$/dt = (1.41 $\pm$ 1.38) $\times$ 10$^{-5}$ d d$^{-1}$, i.e. the period is constant for all practical purposes (Fig.4).

We find that the eclipses depth changes linearly with time and in opposite way on the 18.5-year interval; the total change of the secondary eclipse depth, viz. about 0.35 mag, is larger than the total change of the primary eclipse depth, viz. about 0.10 mag (Fig.5).

The long cycle is revealed in the Weighted Wavelet Z transform (WWZ) as defined by @1996AJ....112.1709F. The WWZ works similarly as the Lomb-Scargle periodogram providing information about the periods of the signal and the time associated to those periods. It is very suitable for the analysis of non- stationary signals and has advantages in the analysis of time-frequency local characteristics. The WWZ shows the long period decreasing from around 900 days to around 800 days through the time baseline (Fig.6).

We fit data segments with order-6 polynomials in order to represent the long cycle. These 