  We report the detection of water absorption features in the dayside spectrum
of the first-known hot Jupiter, 51 Peg b, confirming the star-planet system to
be a double-lined spectroscopic binary. We used high-resolution (R~100,000),
3.2 micron spectra taken with CRIRES/VLT to trace the radial-velocity shift of
the water features in the planet's dayside atmosphere during 4 hours of its
4.23-day orbit after superior conjunction. We detect the signature of molecular
absorption by water at a significance of 5.6 sigma at a systemic velocity of
Vsys=-33+/-2 km/s, coincident with the host star, with a corresponding orbital
velocity Kp = 133^+4.3_-3.5 km/s. This translates directly to a planet mass of
Mp=0.476^+0.032_-0.031MJ, placing it at the transition boundary between Jovian
and Neptunian worlds. We determine upper and lower limits on the orbital
inclination of the system of 70<i (deg)<82.2. We also provide an updated
orbital solution for 51 Peg b, using an extensive set of 639 stellar radial
velocities measured between 1994 and 2013, finding no significant evidence of
an eccentric orbit. We find no evidence of significant absorption or emission
from other major carbon-bearing molecules of the planet, including methane and
carbon dioxide. The atmosphere is non-inverted in the temperature-pressure
region probed by these observations. The deepest absorption lines reach an
observed relative contrast of 0.9x10^-3 with respect to the host star continuum
flux, at an angular separation of 3 milliarcseconds. This work is consistent
with a previous tentative report of K-band molecular absorption for 51 Peg b by
Brogi et al. (2013).
