  We report the detection of the emission line He II 4686 A in eta Carinae. The
equivalent width of this line is ~100 mA along most of the 5.5-yr cycle and
jumps to ~900 mA just before phase 1.0, followed by a brief disappearance. The
similarity between the intensity variations of this line and of the X-ray light
curve is remarkable, suggesting that they are physically connected. We show
that the number of ionizing photons in the ultraviolet and soft X-rays,
expected to be emitted in the shock wave from the colliding winds, is of the
order of magnitude required to produce the He II emission via photoionization.
  The emission is clearly blueshifted when the line is strong. The radial
velocity of the line is generally -100 Km/s, decreases steadily just before the
event, and reaches -400 Km/s at ph = 1.001. At this point, the velocity
gradient suddenly changes sign, at the same time that the emission intensity
drops to nearly zero. Possible scenarios for explaining this emission are
briefly discussed. The timing of the peak of He II intensity is likely to be
associated to the periastron and may be a reliable fiduciary mark, important
for constraining the orbital parameters.
