We present new spectroscopic and photometric observations of the HAT-P-1 planetary system. Spectra obtained during three transits exhibit the Rossiter-McLaughlin effect, allowing us to measure the angle between the sky projections of the stellar spin axis and orbit normal, \lambda = 3.7 +/- 2.1 degrees. The small value of \lambda for this and other systems suggests that the dominant planet migration mechanism preserves spin-orbit alignment. Using two new transit light curves, we refine the transit ephemeris and reduce the uncertainty in the orbital period by an order of magnitude. We find a upper limit on the orbital eccentricity of 0.067, with 99% confidence, by combining our new radial-velocity measurements with those obtained previously.
@article{arxiv.0806.1734,
title = {Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b},
author = {John A. Johnson and Joshua N. Winn and Norio Narita and Keigo Enya and Peter K. G. Williams and Geoffrey W. Marcy and Bun'ei Sato and Yasuhiro Ohta and Atsushi Taruya and Yasushi Suto and Edwin L. Turner and Gaspar Bakos and R. Paul Butler and Steven S. Vogt and Wako Aoki and Motohide Tamura and Toru Yamada and Yuzuru Yoshii and Marton Hidas},
journal= {arXiv preprint arXiv:0806.1734},
year = {2009}
}