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WASP-23b: a transiting hot Jupiter around a K dwarf and its Rossiter-McLaughlin effect

A. H. M. J. Triaud, D. Queloz, C. Hellier, M. Gillon, B. Smalley, L. Hebb, A. Collier Cameron, D. R. Anderson, I. Boisse, G. Hébrard, E. Jehin, T. Lister, C. Lovis, P. F. L. Maxted, F. Pepe, D. Pollacco, D. Ségransan, E. Simpson, S. Udry, R. G. West

We report the discovery of a new transiting planet in the Southern Hemisphere. It has been found by the WASP-south transit survey and confirmed photometrically and spectroscopically by the 1.2m Swiss Euler telescope, LCOGT 2m Faulkes South Telescope, the 60 cm TRAPPIST telescope and the ESO 3.6m telescope. The orbital period of the planet is 2.94 days. We find it is a gas giant with a mass of 0.88 ± 0.10 MJ and a radius estimated at 0.96 ± 0.05 RJ. We have also obtained spectra during transit with the HARPS spectrograph and detect the Rossiter-McLaughlin effect despite its small amplitude. Because of the low signal to noise of the effect and of a small impact parameter we cannot place a constraint on the projected spin-orbit angle. We find two conflicting values for the stellar rotation. Our determination, via spectral line broadening gives v sin I = 2.2 ± 0.3 km s-1, while another method, based on the activity level using the index log R'HK, gives an equatorial rotation velocity of only v = 1.35±0.20 km s-1. Using these as priors in our analysis, the planet could either be misaligned or aligned. This should send strong warnings regarding the use of such priors. There is no evidence for eccentricity nor of any radial velocity drift with time.

binaries: eclipsing - planetary systems - stars: individual: WASP-23 - techniques: spectroscopic - techniques: photometric - stars: rotation

Using WASP-South photometric observations confirmed with LCOGT Faulkes South Telescope, the 60 cm TRAPPIST telescope, the CORALIE spectrograph and the camera from the Swiss 1.2m Euler Telescope placed at La Silla, Chile, as well as with the HARPS spectrograph, mounted on the ESO 3.6 m, also at La Silla, under proposal 084.C-0185. The data is publicly available at the CDS Strasbourg and on demand to the main author.
RV data is only available at the CDS via anonymous ftp to ( or via
Appendix is available in electronic form at

Astronomy & Astrophysics
Volume 531, Number of pages A24_1
2011 July

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia