Period switching in the symbiotic star BX Mon

We report on a detailed analysis of the optical light curve of the symbiotic system BX Mon, the data of which were gathered from the literature. The light curve covers the period December 1889 March 2009, with a gap of no observations between March 1940 and February 1972. The light curve is characterized by strong oscillations of peak to peak amplitude of 2 to more than 3 magnitudes. Before the gap the fluctuations were modulated mainly by a period Pa=1373 d, the diurnal cycle of an observer on the surface of the rotating M giant, whose sun is the hot component. After the gap the dominant periodicity is Pb=1256 d, which is the binary period of the system. Higher harmonics as well as a few beats of the two major periodicities can also be identified in the light curve. We identify one of the beat periods, Pr=656 d, as the sidereal rotation period of the giant component of the system. The period switching that took place during the gap in the observations was possibly associated with a certain cataclysmic event, hints of which may be recognized in the LC in the first 11 years after the gap. We suggest that the origin of the major oscillations is in periodic episodes of mass accretion from the M giant onto the hot component of the system. The event of the period switching is possibly related to an intensive magnetic activity in the outer layers of the giant star. [abridged]