We present high-precision photometric observations of the transiting exoplanets HAT-P-40b and HAT-P-51b by the Rozhen 2-m telescope. The newly-observed transit of HAT-P-40b is the first one with a complete curve. The orbital periods of the two targets were improved. We modeled the observed transits and found bigger stellar radii than those derived from the stellar models. The planet radii of HAT-P-40b and HAT-P-51b obtained from our transit solutions are bigger than the values calculated by the empirical relations for Jupiter-mass and Saturn-mass planets respectively. Their values reveal the highly-inflated nature of the two targets, especially that of HAT-P-51b. We established that the best transit solutions correspond to a quadratic limb-darkening law. The fitted limb-darkening coefficients of HAT-P-40 are close to the theoretical ones while those of HAT-P-51 are a little different. The precise astrometric Gaia distances of the two targets are smaller by 6%-7% than the calculated values from the stellar models. We propose the Gaia distances to be used for improvement of the stellar models as well as for more reliable calculation of the parameters of the known exoplanets.

我们展示了 Rozhen 2 米望远镜对凌日系外行星 HAT-P-40b 和 HAT-P-51b 的高精度光度观测。新观测到的 HAT-P-40b 凌日是第一个具有完整曲线的凌日。两个目标的轨道周期得到改善。我们对观测到的凌日进行了建模，发现恒星半径比从恒星模型中得出的半径更大。从我们的凌日解决方案中获得的 HAT-P-40b 和 HAT-P-51b 的行星半径分别大于木星质量和土星质量行星的经验关系式计算的值。它们的值揭示了这两个目标的高度膨胀特性，尤其是 HAT-P-51b。我们确定最佳的过境解决方案对应于二次肢体变暗定律。HAT-P-40 拟合的肢体变暗系数接近理论值，而 HAT-P-51 则略有不同。精确的天体测量两个目标的盖亚距离比恒星模型的计算值小 6%-7%。我们建议将盖亚距离用于改进恒星模型以及更可靠地计算已知系外行星的参数。