The radii of giant planets , as measured from transit observations , may vary with wavelength due to Rayleigh scattering or variations in opacity .
Such an effect is predicted to be large enough to detect using ground-based observations at multiple wavelengths .
We present defocussed photometry of a transit in the HAT-P-5 system , obtained simultaneously through Strömgren u , Gunn g and r , and Johnson I filters .
Two more transit events were observed through a Gunn r filter .
We detect a substantially larger planetary radius in u , but the effect is greater than predicted using theoretical model atmospheres of gaseous planets .
This phenomenon is most likely to be due to systematic errors present in the u -band photometry , stemming from variations in the transparency of Earth ’ s atmosphere at these short wavelengths .
We use our data to calculate an improved orbital ephemeris and to refine the measured physical properties of the system .
The planet HAT-P-5 b has a mass of 1.06 \pm 0.11 \pm 0.01 { M } _ { Jup } and a radius of 1.252 \pm 0.042 \pm 0.008 { R } _ { Jup } ( statistical and systematic errors respectively ) , making it slightly larger than expected according to standard models of coreless gas-giant planets .
Its equilibrium temperature of 1517 \pm 29 K is within 60 K of that of the extensively-studied planet HD 209458 b .