  CONTEXT: Accurate physical properties of eclipsing stars provide important
constraints on models of stellar structure and evolution, especially when
combined with spectroscopic information on their chemical composition.
Empirical calibrations of the data also lead to accurate mass and radius
estimates for exoplanet host stars. Finally, accurate data for unusual stellar
subtypes, such as Am stars, also help to unravel the cause(s) of their
peculiarities. AIMS: We aim to determine the masses, radii, effective
temperatures, detailed chemical composition and rotational speeds for the
Am-type eclipsing binaries SW CMa (A4-5m) and HW CMa (A6m) and compare them
with similar normal stars. METHODS: Accurate radial velocities from the Digital
Speedometers of the Harvard-Smithsonian Center for Astrophysics were combined
with previously published uvby photometry to determine precise physical
parameters for the four stars. A detailed abundance analysis was performed from
high-resolution spectra obtained with the Nordic Optical Telescope (La Palma).
RESULTS: We find the masses of the (relatively evolved) stars in SW CMa to be
2.10 and 2.24 solar masses, with radii of 2.50 and 3.01 solar radii, while the
(essentially zero-age) stars in HW CMa have masses of 1.72 and 1.78 solar
masses, radii of 1.64 and 1.66 solar radii -- all with errors well below 2%.
Detailed atmospheric abundances for one or both components were determined for
14 elements in SW CMa ([Fe/H] = +0.49/+0.61 dex) and 16 in HW CMa ([Fe/H] =
+0.33/+0.32 dex); both abundance patterns are characteristic of metallic-line
stars. Both systems are well fit by current stellar evolution models for
assumed bulk abundances of [Fe/H] = +0.05 and +0.23, respectively ([alpha/Fe] =
0.0), and ages of about 700 Myr and 160 Myr.
