  DW Carinae is a close but detached early B-type eclipsing binary in the young
open cluster Cr 228. We have measured accurate physical properties of its
components (masses and radii to 1%) and used these to derive the age,
metallicity and distance of Cr 228. The rotational velocities of both
components of DW Car are high, so we have investigated the performance of
double-Gaussian fitting, one- and two-dimensional cross-correlation and
spectral disentangling for measuring radial velocities in the presence of
strong line blending. Gaussian and cross-correlation analyses require
substantial corrections for the effects of line blending, which are only
partially successful for cross-correlation. Disentangling is to be preferred
because it does not assume anything about the shapes of spectral lines, and is
not significantly affected by blending. Complete Stromgren uvby light curves
have been obtained and accurate radii have been measured from them using the
Wilson-Devinney program, constrained by an accurate spectroscopic light ratio.
The Teffs and reddening have been found from Stromgren photometric
calibrations. The masses and radii of DW Car are M_A = 11.34 +/- 0.12 Msun, R_A
= 4.558 +/- 0.045 Rsun, M_B = 10.63 +/- 0.14 Msun and R_B = 4.297 +/- 0.055
Rsun. The Teffs are Teff_A = 27900 +/- 1000 K and Teff_B = 26500 +/- 1000 K,
and the reddening is E(b-y) = 0.18 +/- 0.02 mag. Using empirical bolometric
corrections we find a distance modulus of 12.24 +/- 0.12 mag for DW Car, which
is in agreement with, and more accurate than, literature values for Cr 228. A
comparison between the properties of DW Car and theoretical predictions is
undertaken in the mass--radius and mass--Teff diagrams. Good agreement is found
for an age of about 6 Myr and a metal abundance of Z approx 0.01 (abridged).
