  FIRST is a prototype instrument built to demonstrate the capabilities of the
pupil remapping technique, using single-mode fibers and working at visible
wavelengths. We report on observations of the binary system Capella at three
epochs over a period of 14 months with FIRST-18 (that recombines 2 sets of 9
fibers) mounted on the 3-m Shane telescope at Lick Observatory. The binary
separation during our observations ranges from 0.8 to 1.2 times the diffraction
limit of the telescope at the central wavelength. We successfully resolved the
Capella binary system with an astrometric precision as good as 1mas under the
best observing conditions. FIRST also gives access to the spectral flux ratio
between the two components directly measured with an unprecedented spectral
resolution (around 300) over the 600-850nm range. In particular, our data allow
to detect the well-known overall slope of the flux ratio spectrum, leading to
an estimation of the pivot wavelength of 0.64+/-0.01um, at which the cooler
component becomes the brightest. Spectral features arising from the difference
in effective temperature (specifically the Halpha line, TiO and CN bands) have
been used to constrain the stellar parameters. The effective temperatures we
derive for both components are slightly lower (5-7%) than the well-established
properties for this system. This difference mainly originates from deeper
molecular features than those predicted by state-of-the-art stellar atmospheric
models, suggesting that molecular line lists used in the photospheric models
are incomplete and/or oscillator strengths are underestimated (most likely
concerning the CN molecule). These results demonstrate the power of FIRST, a
fibered pupil remapping based instrument, in terms of high angular resolution
and show that the direct measurement of the spectral flux ratio provides
valuable information to characterize little known companions.
