NEW MASSES FOR THE O-TYPE BINARY DH CEPHEI, AND THE TEMPERATURES OF O-STARS

Spectroscopic observations, secured in 1973 and 1991, are analysed tog ether with data from Sturm & Simon (1993) and Penny et al. (1996) to y ield a new determination of the masses of the O+O-type binary system D H Cep. The masses are 32.7 +/- 1.7 M. for the O5.5V primary and 29.6 /- 1.6 M. for the O6.5V secondary. Analysis of the light curves from L ines et al. (1986), coupled with temperature and flux ratios from Stur m & Simon, results in revised absolute parameters with final masses in creased by 10% relative to those determined by Sturm & Simon. These re vised parameters are in excellent accord with the evolutionary models of Schaller et al. (1992), at an age of 1.5 Myr. In the context of tho se evolutionary models, and the work of Schonberner & Harmanec (1995) on lower-mass binaries, we discuss the properties of 6 O to B0 detache d binaries including DH Cep. The masses and radii for the components o f these systems are all in remarkable agreement with the Schaller et a l. models of solar composition. The O star temperatures need to be red uced by an average similar to 1000 K to achieve agreement in T-eff wit h the models, but this averaged value may hide the need for substantia l reductions (similar to 2500 K) around O8. We note also some discrepa ncies between derived visual absolute magnitudes, and the M(upsilon)-s pectral type relationship for O stars compiled by Schmidt-Kaler (1982) .