New masses for the O-type binary DH Cephei, and the temperatures of O-stars.

Spectroscopic observations, secured in 1973 and 1991, are analysed together with data from Sturm & Simon (1993) and Penny et al. (1996) to yield a new determination of the masses of the O+O-type binary system DH Cep. The masses are 32.7+/-1.7M_sun_ for the O5.5V primary and 29.6+/-1.6M_sun_ for the O6.5V secondary. Analysis of the light curves from Lines et al. (1986), coupled with temperature and flux ratios from Sturm & Simon, results in revised absolute parameters with final masses increased by 10% relative to those determined by Sturm & Simon. These revised parameters are in excellent accord with the evolutionary models of Schaller et al. (1992), at an age of 1.5Myr. In the context of those evolutionary models, and the work of Schoenberner & Harmanec (1995) on lower-mass binaries, we discuss the properties of 6 O to B0 detached binaries including DH Cep. The masses and radii for the components of these systems are all in remarkable agreement with the Schaller et al. models of solar composition. The O star temperatures need to be reduced by an average ~1000K to achieve agreement in T_eff_ with the models, but this averaged value may hide the need for substantial reductions (~2500K) around O8. We note also some discrepancies between derived visual absolute magnitudes, and the M_v_-spectral type relationship for O stars compiled by Schmidt-Kaler (1982).