  The bright binary system YZ Cassiopeiae is a remarkable laboratory for
studying the Am phenomenon. It consists of a metallic-lined A2 star and an F2
dwarf on a circular orbit, which undergo total and annular eclipses. We present
an analysis of 15 published light curves and 42 new high-quality echelle
spectra, resulting in measurements of the masses, radii, effective temperatures
and photospheric chemical abundances of the two stars. The masses and radii are
measured to 0.5% precision: M_A = 2.263 +/- 0.012 Msun, M_B = 1.325 +/- 0.007
Msun, R_A = 2.525 +/- 0.011 Rsun and R_B = 1.331 +/- 0.006 Rsun. We determine
the abundance of 20 elements for the primary star, of which all except scandium
are super-solar by up to 1 dex. The temperature of this star (9520 +/- 120 K)
makes it one of the hottest Am stars. We also measure the abundances of 25
elements for its companion (Teff = 6880 +/- 240 K), finding all to be solar or
slightly above solar. The photospheric abundances of the secondary star should
be representative of the bulk composition of both stars. Theoretical stellar
evolutionary models are unable to match these properties: the masses, radii and
temperatures imply a half-solar chemical composition (Z = 0.009 +/- 0.003) and
an age of 490-550 Myr. YZ Cas therefore presents a challenge to stellar
evolutionary theory.
