All Figures

	Figure 1: Long-term lightcurve of AT Cnc. This lightcurve is obtained from the visual observations of VSNET network observers. Crosses and "v'' symbols represent positive observations and upper limits, respectively.
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In the text

	Figure 2: Differential lightcurves of AT Cnc. The magnitudes are averaged over 152-s time intervals. The rms noise of the lightcurves is 4-6 mmag.
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In the text

	Figure 3: Amplitude spectra of AT Cnc. Most of them display the prominent peaks at the second and third frequency components which indicate an oscillation at a frequency of  mHz. Some amplitude spectra reveal peaks at the fifth and sixth frequency components. These peaks may indicate an oscillation at a frequency of  mHz. Dotted lines mark the corresponding frequencies.
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In the text

	Figure 4: Average power spectrum of AT Cnc. Two large peaks at frequencies of and  mHz are visible in this power spectrum.
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In the text

Figure 5: Fourier power spectra of AT Cnc that are calculated using the data divided in two groups and using all the data together. The inserted frames represent the window functions calculated from sinusoidal signals sampled exactly as the real data. The two groups of the data reveal two nearly coherent oscillations at frequencies (3 February-20 February) and  mHz (26 February-10 March). All the data sets together reveal that these oscillations have poor coherence during the whole observational time. The principal peaks are labeled with "F'' and one-day aliases arelabeled with "A''.

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In the text

Figure 6: SWF power spectra of AT Cnc that are calculated using a sine wave fit to folded lightcurves. Steps in change of trial frequencies are 0.00005 mHz. The forms of these power spectra in the frequency domain in the range 0.03-0.09 mHz are similar to the forms of the Fourier power spectra (Fig. 5). In addition, the SWF power spectra in the upper and lower frames reveal a weakly coherent oscillation at a frequency of or  mHz.

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In the text

Figure 7: SWF power spectra of AT Cnc that are obtained by changing the time of the beginning of the observations of each night randomly with a standard deviation of 10%, 20%, 30% and 40% of the observed period which equals 4.7 h. The upper frame presents the SWF power spectrum without changes of the observational time. This numerical experiment shows that the observed oscillation is poorly coherent with a phase wobbling of about 10%-20% of the oscillation cycle.

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In the text

	Figure 8: Lightcurves of AT Cnc that are folded with a period of 4.65 h for the data obtained in the time interval between 3 February and 20 February (the frame a)) and with a period of 4.74 h for the data obtained in the time interval between 26 February and 10 March (the frame b)). Ten phase bins are used for each folded lightcurve.
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In the text

	Figure 9: Complete differential lightcurve of AT Cnc.
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In the text

	Figure 10: Power spectrum of the complete differential lightcurve.
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In the text