  We present a detailed determination of the astrophysical parameters of the
chromospherically active binary star EI Eridani. Our new radial velocities
allow to improve the set of orbital elements and reveal long-term variations of
the barycentric velocity. A possible third-body orbit with a period of
approximately 19 years is presented. Absolute parameters are determined in
combination with the Hipparcos parallax. EI Eri's inclination angle of the
rotational axis is confined to 56.0 plus/minus 4.5 degrees, its luminosity
class IV is confirmed by its radius of 2.37 plus/minus 0.12 R_Sun. A comparison
to theoretical stellar evolutionary tracks suggests a mass of 1.09 plus/minus
0.05 M_Sun and an age of approximately 6.15 Gyr. The present investigation is
the basis of our long-term Doppler imaging study of its stellar surface.
