Steps towards a solution of the FS Aurigae puzzle - I. Multicolour high-speed photometry with ULTRACAM

We present the analysis of high-speed photometric observations of FS Aurigae taken with the 4.2-m William Herschel Telescope and the high-speed camera ULTRACAM in late 2003. These observations were intended to determine whether there was any evidence of photometric variability with a period in the range 50-100 s. The discovery of such variations would help to explain the existence, in FS Aur, of the very coherent photometric period of 205.5 min that exceeds the spectroscopic period by 2.4 times. Such a discrepancy in the photometric and spectroscopic periods is unusual for a low-mass binary system that is unambiguously identified as a cataclysmic variable. Using various methods, including wavelet analysis, we found that with exception of the 205.5-min periodicity, the main characteristic of variability of FS Aur is usual flickering and quasi-periodic oscillations. However, we detected variability with a period of ∼101 and/or ∼105 s, seen for a short time every half of the orbital period. These oscillations may be associated with the spin period of the white dwarf (WD), not ruling out the possibility that we are observing a processing rapidly rotating WD in FS Aur.