  The beta Cephei star delta Ceti was considered one of the few monoperiodic
variables in the class. Despite (or perhaps because of) its apparently simple
oscillation spectrum, it has been challenging and controversial to identify
this star's pulsation mode and constrain its physical parameters seismically.
Broadband time-resolved photometry of delta Ceti spanning 18.7 days with a duty
cycle of about 65% obtained by the MOST (Microvariability & Oscillations of
STars) satellite -- the first scientific observations ever obtained by MOST --
reveals that the star is actually multiperiodic. Besides the well-known
dominant frequency of f1 = 6.205886/d, we have discovered in the MOST data its
first harmonic 2f1 and three other frequencies (f2 = 3.737/d, f3 = 3.673/d and
f4 = 0.318/d), all detected with S/N > 4. In retrospect, f2 was also present in
archival spectral line profile data but at lower S/N. We present seismic models
whose modes match exactly the frequencies f1 and f2. Only one model falls
within the common part of the error boxes of the star's observed surface
gravity and effective temperature from photometry and spectroscopy. In this
model, f1 is the radial (l = 0) first overtone and f2 is the g2 (l = 2, m = 0)
mode. This model has a mass of 10.2+/-0.2 Msun and an age of 17.9+/-0.3 million
years, making delta Ceti an evolved beta Cephei star. If f2 and f3 are
rotationally split components of the same g2 mode, then the star's equatorial
rotation velocity is either 27.6 km/s or half this value. Given its vsini of
about 1 km/s, this implies we are seeing delta Ceti nearly pole-on.
