The 'WN + WC' Wolf-Rayet stars MR 111 and GP Cep - spectrum binaries or missing links?

MR 111 and GP Cep are two Wolf-Rayet stars classified as 'WN + WC', showing emission features of both the nitrogen and carbon sequences. They are also both known spectroscopic binaries, providing an excellent opportunity to determine whether the WN and WC features arise in two stars or in a single 'transition' object whose composition is intermediate between that of WN and WC stars. A radial velocity study of the C IV 5806A and N IV 7107A lines shows that these two lines vary in phase, and that therefore both sets of emission lines are produced in a single object in each of these two systems. In addition, an orbit solution is given for MR 111. If the mass of the W-R star is like that known in other binary systems, then the mass of the as-yet undetected companion is at least 15 solar masses, consistent with that of other W-R binaries. The mass of companion must be greater than that of the W-R component, unless the W-R star is very massive (m greater than 40 solar masses). 21 refs