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We report the results of the first long-term (1990-2014) opticalspectro-photometric monitoring of a binary black hole candidate QSO E1821+643,a low-redshift high-luminosity radio-quiet quasar. In the monitored period thecontinua and H$\gamma$ fluxes changed for around two times, while the H$\beta$flux changed around 1.4 times. We found the periodical variations in thephotometric flux with the periods of 1200, 1850 and 4000 days, and 4500 daysperiodicity in the spectroscopic variations. However, the periodicity of4000-4500 days covers only one cycle of variation and should be confirmed witha longer monitoring campaign. There is an indication of the period around 1300days in the spectroscopic light curves, but with small significance level,while the 1850 days period could not be clearly identified in the spectroscopiclight curves. The line profiles have not significantly changed, showing animportant red asymmetry and broad line peak redshifted around +1000 kms$^{-1}$. However, H$\beta$ shows broader mean profile and has a largertime-lag ($\tau\sim120$ days) than H$\gamma$ ($\tau\sim60$ days). We estimatethat the mass of the black hole is $\sim2.6\times10^9\rm M_\odot$. The obtainedresults are discussed in the frame of the binary black hole hypothesis. Toexplain the periodicity in the flux variability and high redshift of broadlines we discuss a scenario where dense gas-rich cloudy-like structures areorbiting around a recoiling black hole.

First long-term optical spectro-photometric monitoring of a binary black hole candidate E1821+643: I. Variability of spectral lines and continuum