We report the results of the first long-term ( 1990–2014 ) optical spectro-photometric monitoring of a binary black hole candidate QSO E1821+643 , a low-redshift high-luminosity radio-quiet quasar .
In the monitored period the continua 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 the photometric flux with the periods of 1200 , 1850 and 4000 days , and 4500 days periodicity in the spectroscopic variations .
However , the periodicity of 4000–4500 days covers only one cycle of variation and should be confirmed with a longer monitoring campaign .
There is an indication of the period around 1300 days in the spectroscopic light curves , but with small significance level , while the 1850 days period could not be clearly identified in the spectroscopic light curves .

The line profiles have not significantly changed , showing an important red asymmetry and broad line peak redshifted around +1000 km s ^ { -1 } .
However , H \beta shows broader mean profile and has a larger time-lag ( \tau \sim 120 days ) than H \gamma ( \tau \sim 60 days ) .
We estimate that the mass of the black hole is \sim 2.6 \times 10 ^ { 9 } M _ { \odot } .

The obtained results are discussed in the frame of the binary black hole hypothesis .
To explain the periodicity in the flux variability and high redshift of broad lines we discuss a scenario where dense gas-rich cloudy-like structures are orbiting around a recoiling black hole .