Superhumps in Cataclysmic Binaries. XXI. HP Librae (=EC 15330−1403)

We report photometry of the helium-dominated cataclysmic variable HP Librae during 1995-2001. The main photometric signal varies between 1118.89 and 1119.14 s, on a timescale of a few years, and displays a waveform characteristic of "superhumps." After subtracting the main signal, we found a weak residual signal at 1102.70±0.05 s, which we interpret as the underlying orbital period of the binary. The full amplitude of this putative orbital variation is just 0.005 mag, the weakest orbital signal yet found in a cataclysmic variable (CV). The 1119 s signal of HP Lib is a superb match to the well-studied 1051 s superhump of AM CVn, the "mother of all helium CVs." The superhump shows no change in amplitude or waveform on any timescale and no essential change in period on timescales shorter than ~3000 cycles. Such great stability makes the star a promising test case for detailed studies of the underlying spiral structure in the disk, the likely cause of superhumps. Comparison of orbital and superhump periods for the family of AM CVn stars supplies evidence that these stars are evolving toward longer orbital period.