Detection of superhumps in the Z Camelopardalis-type dwarf nova AT Cnc at standstill

We present results of our photometry of the Z Camelopardalis-type dwarf nova AT Cnc. The observational data were obtained during 13 nights in February and March 2003 when AT Cue was in its long standstill. Two sets of our data reveal brightness variations with quasi-periods of (4.65 +/- 0.02) and of (4.74 +/- 0.02) h. The semiamplitude of these variations was observed in the range 5-9 mmag, showing changes from night to night. This signal varies in period and in phase on a time-scale of weeks, as is typical of superhumps. A comparison with the orbital period obtained by Nogami et al. (1999) from the radial velocity measurements of AT Cnc, which equals (4.826 +/- 0.014) h, shows that this signal can be a negative superhump since the average period of brightness variations is approximately 3% shorter than the orbital period. These results make AT Cue a permanent superhump system with a rather large orbital period and a large mass ratio. This is also the first detection of superhumps in a Z Camelopardalis-type system. In addition, we have found signs of an unstable signal with a period of about 2.3 h, which may represent the second harmonic of the 4.7-h signal or the second orbital sideband of the wobble frequency of the tilted precessing accretion disc. The average power spectrum of AT Cue reveals a broad hump at frequencies in the range 0.4-0.7 mHz, which is evidence of quasiperiodic oscillations. The hump crest corresponds to a period of 1700-1800 s, which is remarkably close to 10% of the orbital period. This may indicate that this system contains a magnetic white dwarf creating the tilted accretion disc.