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

Abstract

We present the analysis of high-speed photometric observations of FS Aur 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 seconds. The discovery of such variations would help to explain 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 an 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-minute 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 sec, seen for a short time every half of the orbital period. These oscillations may be associated with the spin period of the white dwarf, not ruling out the possibility that we are observing a precessing rapidly rotating white dwarf in FS Aur.