  MOST time-series photometry of mu Eri, an SB1 eclipsing binary with a
rapidly-rotating SPB primary, is reported and analyzed. The analysis yields a
number of sinusoidal terms, mainly due to the intrinsic variation of the
primary, and the eclipse light-curve. New radial-velocity observations are
presented and used to compute parameters of a spectroscopic orbit. Frequency
analysis of the radial-velocity residuals from the spectroscopic orbital
solution fails to uncover periodic variations with amplitudes greater than 2
km/s. A Rossiter-McLaughlin anomaly is detected from observations covering
ingress.
  From archival photometric indices and the revised Hipparcos parallax we
derive the primary's effective temperature, surface gravity, bolometric
correction, and the luminosity. An analysis of a high signal-to-noise
spectrogram yields the effective temperature and surface gravity in good
agreement with the photometric values. From the same spectrogram, we determine
the abundance of He, C, N, O, Ne, Mg, Al, Si, P, S, Cl, and Fe.
  The eclipse light-curve is solved by means of EBOP. For a range of mass of
the primary, a value of mean density, very nearly independent of assumed mass,
is computed from the parameters of the system. Contrary to a recent report,
this value is approximately equal to the mean density obtained from the star's
effective temperature and luminosity.
  Despite limited frequency resolution of the MOST data, we were able to
recover the closely-spaced SPB frequency quadruplet discovered from the ground
in 2002-2004. The other two SPB terms seen from the ground were also recovered.
Moreover, our analysis of the MOST data adds 15 low-amplitude SPB terms with
frequencies ranging from 0.109 c/d to 2.786 c/d.
