5 Short period oscillations

In the time series of 1995 observations, especially those in the first night (May 22), a fluctuation with an average amplitude of about 0.05 mag can be seen (Fig. 5). In order to make sure that the fluctuation is intrinsic, we subtract the variation caused by orbital motion of the binary from the time series. The binned light curves in Fig. 2 result from the orbital motion with an effect of starspots (if any). Having subtracted the interpolated values of the smooth light curves, the residual series have been shown in Fig. 6. In the corner of each panel the letter refers to the bandpass used in the observation and the number 1 or 2 stands for the first or second night when the observation was performed. From the figure we more clearly find the fast variation with amplitude much larger than the probable errors of the observations, especially for V bandpass of the first night observation. Moreover, it seems that the amplitude of the fluctuation is larger around the phase zero where the secondary transits the primary. If it is true, one can look for the source of the undulation from the secondary. It is most likely that the source may be the starspot(s) which caused large light and color variations and the undulation.

  

Figure 6: The residual series in which the eclipse effect has been removed. In the upper panels (May 22 observation) we clearly find that the fluctuation has a larger amplitude, particularly around phase 1, suggesting that it should be intrinsic

FFT analysis for the subtracted-data of the observation on 1995 May 22 indicates that there may exist the spikes at frequencies of 0.0003 and 0.008 Hz. The additional spikes at 0.0049, 0.0052 Hz in blue light and a spike around 0.005 Hz and a small hump around 0.0027 Hz in yellow light can be seen (Fig. 7). We appear to have detected the oscillations with periods of 55 and 2 minutes (coferent time is only days) and the quasi-periodic oscillations with periods around 6 and 3 minutes. The FFT spectra of May 23 observation are noisier. The spikes and the hump are drowned out, which is consistent with the lower dispersion in the lower panels of Fig. 6. However, a common feature in the FFT spectra of two night observations is the rise in power towards lower frequencies.

In order to test the reality of the peaks, we employed the CLEAN method to analyze the same data and we obtained the same results. Each panel of Fig. 7 looks like power spectra of cataclysmic variables of which some show quasi- periodic and/or coherent oscillation and the most have nonperiodic, predominantly low-frequency flickering (e.g. Robinson & Nather 1979; Patterson 1981). To our knowledge, these types of variations have not been detected in other W UMa stars. The exciting and attenuating mechanisms of the oscillations should be investigated.

  

Figure 7: The power spectra of the data shown in Fig. 6, which indicates that some short period oscillations seem to be detected on May 22, 1995 (two upper panels)