  We report the discovery of HAT-P-31b, a transiting exoplanet orbiting the
V=11.660 dwarf star GSC 2099-00908. HAT-P-31b is the first HAT planet
discovered without any follow-up photometry, demonstrating the feasibility of a
new mode of operation for the HATNet project. The 2.17 Mj, 1.1Rj planet has a
period P = 5.0054 days and maintains an unusually high eccentricity of e =
0.2450+/-0.0045, determined through Keck, FIES and Subaru high precision radial
velocities. Detailed modeling of the radial velocities indicates an additional
quadratic residual trend in the data detected to very high confidence. We
interpret this trend as a long-period outer companion, HAT-P-31c, of minimum
mass 3.4Mj and period >2.8 years. Since current RVs span less than half an
orbital period, we are unable to determine the properties of HAT-P-31c to high
confidence. However, dynamical simulations of two possible configurations show
that orbital stability is to be expected. Further, if HAT-P-31c has non-zero
eccentricity, our simulations show that the eccentricity of HAT-P-31b is
actively driven by the presence of c, making HAT-P-31 a potentially intriguing
dynamical laboratory.
