Superhumps in cataclysmic binaries. XVII. AM Canum Venaticorum

We report new photometry of the cataclysmic variable AM Canum Venaticorum, comprising 670 hr over 227 nights during 1992-1999. The data demonstrate co nclusively 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 similar to 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 h ypothesized to explain the spectroscopic variations. The behavior of the 1051 s signal is consistent with the hypothesis of a su perhump origin, and the discovery of a signal at the presumed P-orb is yet another strong argument for this hypothesis. Apparently superhumps can manu facture 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 th e latter correspond to the periods of apsidal advance and nodal regression. The former is steadily present, evidently producing the well-known superhu mp. The signatures of nodal regression, primarily the 1011.4 s "negative su perhump," are more variable. The sideband structure of the harmonics obeys firm selection rules, which may reveal intricate details of structure in th e accretion disk. And somehow all these exotic clocks manage to coexist and remain relatively stable, despite being putatively seated in the relativel y sloppy structure of a disk, highly sheared and unsupported by pressure!.