1999PASP..111.1281S

We report new photometry of the cataclysmic variable AM Canum Venaticorum, comprising 670 hr over 227 nights during 1992-1999. The data demonstrate conclusively that (1) the fundamental period is 1051.2 s, with a well-defined waveform which has not changed during the four decades of study; (2) this period wanders erratically by ∼0.2 s on a timescale of 3-6 months; and (3) the light curve contains a rich spectrum of periodic signals, at least 20 of them. We also report a secure detection of a photometric signal at 1028.7322±0.0003 s, consistent with the orbital period previously hypothesized to explain the spectroscopic variations.

The behavior of the 1051 s signal is consistent with the hypothesis of a superhump origin, and the discovery of a signal at the presumed P_orb is yet another strong argument for this hypothesis. Apparently superhumps can manufacture a very complex spectrum of photometric periods, and they can remain stable for many thousands of cycles. But most are harmonics and sidebands of the truly fundamental clocks in the binary, at P_orb and the disk's two likely periods of precession–at 13.36 and 16.69 hr. It is plausible that the latter correspond to the periods of apsidal advance and nodal regression. The former is steadily present, evidently producing the well-known superhump. The signatures of nodal regression, primarily the 1011.4 s ``negative superhump'', are more variable. The sideband structure of the harmonics obeys firm selection rules, which may reveal intricate details of structure in the accretion disk. And somehow all these exotic clocks manage to coexist and remain relatively stable, despite being putatively seated in the relatively sloppy structure of a disk, highly sheared and unsupported by pressure!