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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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Title: The 0.5 Jupiter mass transiting exoplanet WASP-13b
Authors: I. Skillen, D. Pollacco, A. Collier Cameron, L. Hebb, E. Simpson, F. Bouchy, D.J. Christian, N.P. Gibson, G. Hébrard, Y.C. Joshi, B. Loeillet, B. Smalley, H.C. Stempels, R.A. Street, S. Udry, R.G. West, D.R. Anderson, S.C.C. Barros, B. Enoch, C.A. Haswell, C. Hellier, K. Horne, J. Irwin, F.P. Keenan, T.A. Lister, P. Maxted, M. Mayor, C. Moutou, A.J. Norton, N. Parley, D. Queloz, R. Ryans, I. Todd, P.J. Wheatley, D.M. Wilson

We report the discovery of WASP-13b, a low-mass M_p = 0.46 ^{+ 0.06}_{- 0.05} Jupiter mass transiting exoplanet with an orbital period of 4.35298 ±0.00004 days. The transit has a depth of 9 mmag, and although our follow-up photometry does not allow us to constrain the impact parameter well (0 < b < 0.46), with radius in the range R_p \sim 1.06 - 1.21 Jupiter radius the location of WASP-13b in the mass-radius plane is nevertheless consistent with H/He-dominated, irradiated, low core mass and core-free theoretical models. The G1V host star is similar to the Sun in mass (M_{*} = 1.03^{+0.11}_ {- 0.09} solar masses) and metallicity ([M/H]=0.0±0.2), but is possibly older (8.5^{+ 5.5}_{- 4.9} Gyr).

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