* Astronomy

Title: High precision transit observations of the exoplanet WASP-13b with the RISE instrument
Authors: S. C. C. Barros (1), D. L. Pollacco (1), N. P. Gibson (2), F. P. Keenan (1), I. Skillen (3), I. A. Steele (4) (1- Queen's University Belfast, 2- University of Oxford, 3-ING, 4-Liverpool John Moores University)

WASP-13b is a sub-Jupiter mass exoplanet orbiting a G1V type star with a period of 4.35 days. The current uncertainty in its impact parameter (0 < b < 0.46) resulted in poorly defined stellar and planetary radii. To better constrain the impact parameter we have obtained high precision transit observations with the RISE instrument mounted on 2.0 m Liverpool Telescope. We present four new transits which are fitted with an MCMC routine to derive accurate system parameters. We found an orbital inclination of 85.2 ±0.3 degrees resulting in stellar and planetary radii of 1.56 ±0.04 solar radii and 1.39 ±0.05 Jupiter radii, respectively. This suggests that the host star has evolved off the main-sequence and is in the shell hydrogen-burning phase. We also discuss how the limb darkening affects the derived system parameters. With a density of 0.17{
ho}J, WASP-13b joins the group of low density planets whose radii are too large to be explained by standard irradiation models. We derive a new ephemeris for the system, T0 = 2455575.5136 ±0.0016 (HJD) and P = 4.353011 ±0.000013 days. The planet equilibrium temperature (Tequ = 1500 K) and the bright host star (V = 10.4 mag) make it a good candidate for follow-up atmospheric studies.

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