  The planet in the WASP-103 system is an excellent candidate for transmission
spectroscopy because of its large radius and high temperature. Application of
this technique found a variation of radius with wavelength which was far too
strong to be explained by scattering processes in the planetary atmosphere. A
faint nearby star was subsequently detected, whose contamination of the transit
light curves might explain this anomaly. We present a reanalysis of published
data in order to characterise the faint star and assess its effect on the
measured transmission spectrum. The faint star has a mass of 0.72 +/- 0.08 Msun
and is almost certainly gravitationally bound to the planetary system. We find
that its effect on the measured physical properties of the planet and host star
is small, amounting to a planetary radius larger by 0.6 sigma and planetary
density smaller by 0.8 sigma. Its influence on the measured transmission
spectrum is much greater: the spectrum now has a minimum around 760 nm and
opacity rises to both bluer and redder wavelengths. It is a poor match to
theoretical spectra and the spectral slope remains too strong for Rayleigh
scattering. The existence of the faint nearby star cannot therefore explain the
measured spectral properties of this hot and inflated planet. We advocate
further observations of the system, both with high spatial resolution in order
to improve the measured properties of the faint star, and with higher spectral
resolution to confirm the anomalous transmission spectrum of the planet.
