We present seven light curves of the exoplanet system HAT-P-3 , taken as part of a transit timing program using the RISE instrument on the Liverpool Telescope .
The light curves are analysed using a Markov-Chain Monte-Carlo algorithm to update the parameters of the system .
The inclination is found to be i = 86.75 ^ { +0.22 } _ { -0.21 } \deg , the planet-star radius ratio to be R _ { p } / R _ { \star } = 0.1098 ^ { +0.0010 } _ { -0.0012 } , and the stellar radius to be R _ { \star } = 0.834 ^ { +0.018 } _ { -0.026 } R _ { \odot } , consistent with previous results but with a significant improvement in the precision .
Central transit times and uncertainties for each light curve are also determined , and a residual permutation algorithm used as an independent check on the errors .
The transit times are found to be consistent with a linear ephemeris , and a new ephemeris is calculated as T _ { c } ( 0 ) = 2454856.70118 \pm 0.00018 HJD and P = 2.899738 \pm 0.000007 days .
Model timing residuals are fitted to the measured timing residuals to place upper mass limits for a hypothetical perturbing planet as a function of the period ratio .
These show that we have probed for planets with masses as low as 0.33 M _ { \oplus } and 1.81 M _ { \oplus } in the interior and exterior 2:1 resonances , respectively , assuming the planets are initially in circular orbits .