  We present ULTRACAM photometry of ES Cet, an ultracompact binary with a 620s
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 semi-degenerate donor star expands in size as it loses mass.
We supplement these ULTRACAM+WHT data with observations made with smaller
telescopes around the world over a nine year 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 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.
