  We present the discovery of the Kepler-19 planetary system, which we first
identified from a 9.3-day periodic transit signal in the Kepler photometry.
From high-resolution spectroscopy of the star, we find a stellar effective
temperature Teff=5541 \pm 60 K, a metallicity [Fe/H]=-0.13 \pm 0.06, and a
surface gravity log(g)=4.59 \pm 0.10. We combine the estimate of Teff and
[Fe/H] with an estimate of the stellar density derived from the photometric
light curve to deduce a stellar mass of M_star = 0.936 \pm 0.040 M_Sun and a
stellar radius of R_star = 0.850 \pm 0.018 R_Sun. We rule out the possibility
that the transits result from an astrophysical false positive by first
identifying the subset of stellar blends that reproduce the precise shape of
the light curve. We conclude that the planetary scenario is more than three
orders of magnitude more likely than a blend. The blend scenario is
independently disfavored by the achromaticity of the transit: we measure a
transit depth with Spitzer at 4.5 {\mu}m of 547+113-110 ppm, consistent with
the depth measured in the Kepler optical bandpass of 567\pm6 ppm. We determine
a physical radius of the planet Kepler-19b of R_p = 2.209 \pm 0.048 R_Earth.
From radial-velocity observations of the star, we find an upper limit on the
planet mass of 20.3 M_Earth, corresponding to a maximum density of 10.4 g
cm^-3. We report a significant sinusoidal deviation of the transit times from a
predicted linear ephemeris, which we conclude is due to an additional
perturbing body in the system. We cannot uniquely determine the orbital
parameters of the perturber, as various dynamical mechanisms match the
amplitude, period, and shape of the transit timing signal and satisfy the host
star's radial velocity limits. However, the perturber in these mechanisms has
period <160 days and mass <6 M_Jup, confirming its planetary nature as
Kepler-19c. [Abridged]
