Evolution of the Orbital Period of the Ultracompact Binary System ZTF J213056.71+442046.5

We have analyzed the change in the period of the ultracompact binary system ZTF J213056.71+442046.5, a source of potentially detectable mHz gravitational waves for planned space laser interferometers. The photometric data cover a time interval of almost 6.5 years and include our own observations carried out with the RC600 telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University and data from the ZTF sky survey. The $$O-C$$ diagram can be described by quadratic light elements that correspond to a rate of period decrease $$dP/dt=(-2.66\pm 0.62)\times 10^{-12}$$ s s $${}^{-1}$$ . This value leads to an almost twofold increase in the expected signal-to-noise ratio for the observations of gravitational waves from this binary system with space laser interferometers.