NLTE analysis of hot binaries

We present quantitative spectroscopic NLTE analyses of the components of well detached early type binaries (DH Cep, Y Cyg, V453 Cyg, and CW Cep). The position of the stars in the logL-logT _eff diagram is discussed. We find significantly higher temperatures for the components of Y Cygni from spectral analysis by means of unblanketed NLTE model photospheres than those given by the orbit analysis. Therefore the comparison with evolutionary tracks yields larger masses. The spectroscopic temperatures of V453 Cygni and CW Cephei agree with the orbit data, but the evolutionary tracks point to larger masses also. However, if we account for some 2000K lower effective temperatures due to line blanketing, the luminosities, temperatures and masses of all stellar components are in good agreement, except for the case of DH Cep.