  Transits in the WASP-57 planetary system have been found to occur half an
hour earlier than expected. We present ten transit light curves from amateur
telescopes, on which this discovery was based, thirteen transit light curves
from professional facilities which confirm and refine this finding, and
high-resolution imaging which show no evidence for nearby companions. We use
these data to determine a new and precise orbital ephemeris, and measure the
physical properties of the system. Our revised orbital period is 4.5s shorter
than found from the discovery data alone, which explains the early occurrence
of the transits. We also find both the star and planet to be larger and less
massive than previously thought. The measured mass and radius of the planet are
now consistent with theoretical models of gas giants containing no
heavy-element core, as expected for the sub-solar metallicity of the host star.
Two transits were observed simultaneously in four passbands. We use the
resulting light curves to measure the planet's radius as a function of
wavelength, finding that our data are sufficient in principle but not in
practise to constrain its atmospheric properties. We conclude with a discussion
of the current and future status of transmission photometry studies for probing
the atmospheres of gas-giant transiting planets.
