  We report the detection of a hot companion of $\eta$ Carinae using high
resolution spectra (905 - 1180 \AA) obtained with the Far Ultraviolet
Spectroscopic Explorer (\fuse) satellite. Observations were obtained at two
epochs of the 2024-day orbit: 2003 June during ingress to the 2003.5 X-ray
eclipse and 2004 April several months after egress. These data show that
essentially all the far-UV flux from \etacar shortward of \lya disappeared at
least two days before the start of the X-ray eclipse (2003 June 29), implying
that the hot companion, \etaB, was also eclipsed by the dense wind or extended
atmosphere of \etaA. Analysis of the far-UV spectrum shows that \etaB is a
luminous hot star. The \nii \wll1084-1086 emission feature suggests that it may
be nitrogen-rich. The observed far-UV flux levels and spectral features,
combined with the timing of their disappearance, is consistent with \etacar\
being a massive binary system.
