The photometric period in ES Ceti

We present ULTRACAM photometry of ES Cet, an ultracompact binary with a 620-s orbital period. The mass transfer in systems such as this one is thought to be driven by gravitational radiation, which causes the binary to evolve to longer periods since the semidegenerate donor star expands in size as it loses mass. We supplement these ULTRACAM+William Herschel Telescope (WHT) data with observations made with smaller telescopes around the world over a 9-yr baseline. All of the observations show variation on the orbital period, and by timing this variation we track the period evolution of this system. We do not detect any significant departure from a linear ephemeris, implying a donor star that is of small mass and close to a fully degenerate state. This finding favours the double white dwarf formation channel for this AM Canum Venaticorum (AM CVn) star. An alternative explanation is that the system is in the relatively short-lived phase in which the mass transfer rate climbs towards its long-term value.