2013A&A...553A..26N

Multi-band observations of planetary transits using the telescope defocus technique may yield high-quality light curves suitable for refining the physical properties of exoplanets even with small or medium size telescopes. Such observations can be used to construct a broad-band transmission spectrum of transiting planets and search for the presence of strong absorbers. We have thoroughly characterised the orbital ephemeris and physical properties of the transiting planet and host star in the WASP-23b system, constructed a broad-band transmission spectrum of WASP-23b and performed a comparative analysis with theoretical models of hot Jupiters. We observed a complete transit of WASP-23b in seven passbands simultaneously, using the GROND instrument on the MPG/ESO 2.2m telescope at La Silla Observatory and telescope defocussing. The optical data were taken in the Sloan g^', r^', i^' and z^' passbands. The resulting light curves are of high quality, with a root-mean-square scatter of the residual as low as 330 parts per million (ppm) in the z^'-band, with a cadence of 90s. Near-infrared data were obtained in the JHK passbands. We performed a MCMC analysis of our photometry plus existing radial velocity data to refine measurements of the ephemeris and physical properties of the WASP-23 system. We constructed a broad-band transmission spectrum of WASP-23b and compared it with a theoretical transmission spectrum of a hot Jupiter. We measured the central transit time with a precision ∼8s. From this and earlier observations we obtain an orbital period of P = 2.9444300 ±0.0000011d. Our analysis also yielded a larger radius and mass for the planet (R_p=1.067^+0.045_–0.038 R_Jup and M_p=0.917^+0.040_–0.039 M_Jup) compared to previous estimates (R_p=0.962^+0.047_–0.056 R_Jup and M_p=0.884^+0.088_–0.094 M_Jup). The derived transmission spectrum is marginally flat, which is not surprising given the limited precision of the measurements for the planetary radius and the poor spectral resolution of the data.