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 4.8 s , versus 67.4 s 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 .