  We present new results from a study of high quality, red spectra of the
massive binary star system HD 1383 (B0.5 Ib + B0.5 Ib). We determined radial
velocities and revised orbital elements (P = 20.28184 +/- 0.0002 d) and made
Doppler tomographic reconstructions of the component spectra. A comparison of
these with model spectra from non-LTE, line blanketed atmospheres indicates
that both stars have almost identical masses (M_2/M_1 = 1.020 +/- 0.014),
temperatures (T_eff = 28000 +/- 1000 K), gravities (log g = 3.25 +/- 0.25), and
projected rotational velocities (V sin i < 30 km/s). We investigate a number of
constraints on the radii and masses of the stars based upon the absence of
eclipses, surface gravity, stellar wind terminal velocity, and probable
location in the Perseus spiral arm of the Galaxy, and these indicate a range in
probable radius and mass of R/R_sun = 14 - 20 and M/M_sun = 16 - 35,
respectively. These values are consistent with model evolutionary masses for
single stars of this temperature and gravity. Both stars are much smaller than
their respective Roche radii, so the system is probably in a pre-contact stage
of evolution. A fit of the system's spectral energy distribution yields a
reddening of E(B-V)=0.55 +/- 0.05 and a ratio of total-to-selective extinction
of R=2.97 +/- 0.15. We find no evidence of H-alpha emission from colliding
stellar winds, which is probably the consequence of the low gas densities in
the colliding winds zone.
