A dynamical explanation for a long-term modulation in the light curve of FS Aurigae

Aims. FS Aur is an unusual cataclysmic variable, which in addition to its spectroscopically determined orbital period of 85.7 min exhibits spectroscopic and photometric periods of 147 and 205 min, correspondingly, which are attributed to the period of the fast rotating and freely precessing magnetic white dwarf. We aim to study the origin of a very long term variability observed in the light curve of FS Aur accumulated by us and in the AAVSO International Database over the past thirteen years. Methods. Discrete Fourier transform algorithm and Scargle-Lomb least-squares spectral analysis were used for the period analysis of the observational data. A 12th-order Runge-Kutta-Nyström integrator was used to conduct numerical simulations of a three-body system that is invoked to explain the observed long-term variability in the FS Aur light curve, which is probably periodic. Results. We report here a newly discovered ~2-mag modulation of the light curve of FS Aur with a ~900-day period. The huge disparity with the previous periods makes this difficult to explain within the confines of the binary system. We propose a mechanism for this longer period: a modulation in the binary eccentricity, produced by the gravitational perturbation of a third object on a larger orbit. We first explore orbits with a period equal to the ~900-day modulation and a high eccentricity that can produce a direct perturbation in the close binary system, and conclude that the shape of the time response does not match the observed light curve. However, putting the perturber on a circular, shorter period orbit at the proper location can induce a long-term secular modulation. Conclusions. There is a tight correlation between the perturber mass and orbital radius with the induced long-term modulation in the central binary. To explain the observed ~900-day period, the perturber must be a substellar mass object with a probable mass between 25 to 64 times that of Jupiter. The presence of the third body in this system may be a clue for understanding other observational peculiarities