Discovery of the 1.80 h spin period of the white dwarf of the symbiotic system BF Cyg

We report on the discovery of a coherent periodicity in the B light curve of the symbiotic star BF Cyg. The signal was detected in some sections of the light curve of the star recorded in the year 2003 as double-hump periodic variations with an amplitude of ?7 mmag. In the year 2004, the signal was also present in only a subsection of the light curve. In that year, the system was about twice as bright and the amplitude of the oscillations was about half of what it was in 2003. In 2004, the cycle structure was of a single hump, the phase of which coincided with the phase of one of the humps in the 2003 cycle. No periodic signal was detected in a third, short series of observations performed in the year 2007, when the star was 3 mag brighter than in 2003. We interpret the periodicity as the spin period of the white dwarf component of this interacting binary system. We suggest that the signal in 2003 originated in two hotspots on or near the surface of the white dwarf most likely around the two antipodes of an oblique dipole magnetic field of this star. Magnetic field lines funnelled accreted matter from the wind of the cool component to the pole areas, where the falling material created the hotspots. This process is apparently intermittent in its nature. In 2004, the activity near only one pole was enhanced enough to raise the signal above the threshold of our detection ability.