Spin-period variations in the intermediate polar RX J2133.7 + 5107

ABSTRACT We report the results of long-term time series photometry on RX J2133.7+5107 (also known as 1RXS J213344.1+510725) obtained at several observatories. Using data taken during 17 yr, we determined the current value of the spin period of 570.811470 s with the formal accuracy of 0.000006 s and a spin-up of the white dwarf with a characteristic time of $1.483(1)\times 10^5$ yr. This is even faster than that reported previously and if confirmed, makes this object have one of the fastest spin-up time-scales of all-known intermediate polars. We derived an improved value of the superhump period of the system to be $0^d.280130(1)$. Superhump maxima timings are moving on the phase curve from season to season, showing non-monotonic changes, without a change in superhump period.