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Two photometric periods in the AM CVn system CP Eridani

Authors

  • E. Armstrong,

    Corresponding author
    1. Department of Physics, University of California, San Diego, La Jolla, CA 92037, USA
    2. Department of Astronomy, Columbia University, Mail Code 5246, 550 West 120th Street, New York, NY 10027, USA
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  • J. Patterson,

    Corresponding author
    1. Department of Astronomy, Columbia University, Mail Code 5246, 550 West 120th Street, New York, NY 10027, USA
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  • J. Kemp

    Corresponding author
    1. Gemini Observatory, Northern Operations Center, 670 North A’ohoku Place, Hilo, HI 96720, USA
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E-mail: earmstrong@physics.ucsd.edu (EA); jop@astro.columbia.edu (JP); jkemp@gemini.edu (JK)

ABSTRACT

We report photometric periodicities in the AM CVn system CP Eridani (CP Eri) of 1716.2 ± 0.2 and 1701.4 ± 0.2 s, obtained while the system was in quiescence. From a second observation obtained during superoutburst, we interpret the 1716-s signal as a superhump period (Psh) and the 1701-s signal as the orbital period (Porb) of the binary. The derived fractional superhump period excess ɛ adds CP Eri to a small collection of AM CVn stars with ɛ measured via time-series photometry of superhump and orbital periods. Plotting ɛ(Porb) for these systems, we find that AM CVn systems may, as expected, be evolving towards longer Porb. We discuss a technique of using ɛ to determine the degree of degeneracy of the donor star in a contact binary, and we show that for the AM CVn systems, the donor stars are well described by mass–radius relations of partially-degenerate objects.

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