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FIRST is a prototype instrument built to demonstrate the capabilities of thepupil remapping technique, using single-mode fibers and working at visiblewavelengths. We report on observations of the binary system Capella at threeepochs over a period of 14 months with FIRST-18 (that recombines 2 sets of 9fibers) mounted on the 3-m Shane telescope at Lick Observatory. The binaryseparation during our observations ranges from 0.8 to 1.2 times the diffractionlimit of the telescope at the central wavelength. We successfully resolved theCapella binary system with an astrometric precision as good as 1mas under thebest observing conditions. FIRST also gives access to the spectral flux ratiobetween the two components directly measured with an unprecedented spectralresolution (around 300) over the 600-850nm range. In particular, our data allowto detect the well-known overall slope of the flux ratio spectrum, leading toan estimation of the pivot wavelength of 0.64+/-0.01um, at which the coolercomponent becomes the brightest. Spectral features arising from the differencein effective temperature (specifically the Halpha line, TiO and CN bands) havebeen used to constrain the stellar parameters. The effective temperatures wederive for both components are slightly lower (5-7%) than the well-establishedproperties for this system. This difference mainly originates from deepermolecular features than those predicted by state-of-the-art stellar atmosphericmodels, suggesting that molecular line lists used in the photospheric modelsare incomplete and/or oscillator strengths are underestimated (most likelyconcerning the CN molecule). These results demonstrate the power of FIRST, afibered pupil remapping based instrument, in terms of high angular resolutionand show that the direct measurement of the spectral flux ratio providesvaluable information to characterize little known companions.

FIRST, a fibered aperture masking instrument II. Spectroscopy of the Capella binary system at the diffraction limit