  We present 17 transit light curves of the ultra-short period planetary system
WASP-103, a strong candidate for the detection of tidally-induced orbital
decay. We use these to establish a high-precision reference epoch for transit
timing studies. The time of the reference transit midpoint is now measured to
an accuracy of 4.8s, versus 67.4s in the discovery paper, aiding future
searches for orbital decay. With the help of published spectroscopic
measurements and theoretical stellar models, we determine the physical
properties of the system to high precision and present a detailed error budget
for these calculations. The planet has a Roche lobe filling factor of 0.58,
leading to a significant asphericity; we correct its measured mass and mean
density for this phenomenon. A high-resolution Lucky Imaging observation shows
no evidence for faint stars close enough to contaminate the point spread
function of WASP-103. Our data were obtained in the Bessell $RI$ and the SDSS
$griz$ passbands and yield a larger planet radius at bluer optical wavelengths,
to a confidence level of 7.3 sigma. Interpreting this as an effect of Rayleigh
scattering in the planetary atmosphere leads to a measurement of the planetary
mass which is too small by a factor of five, implying that Rayleigh scattering
is not the main cause of the variation of radius with wavelength.
