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

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 years, we determined the current value of the spin period of $570.811470$ seconds with the formal accuracy of $0.000006$ seconds and a spin-up of the white dwarf with a characteristic time of $1.483(1)\times10^5$ years. This is even faster than that reported previously and, if confirmed, makes this object have one of the fastest spin-up timescales 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.