MASSIVE CLOSE BINARIES - OBSERVATIONAL CHARACTERISTICS

Theoretically predicted evolutionary phases of massive close binaries are compared with the observations. For the evolution up to the High-M ass X-ray Binary (HMXB) phase there is fair agreement between theory a nd observation. Beyond the HMXB phase there is much uncertainty. Notab ly it is puzzling why we observe so few systems consisting of a helium star and a neutron star (Cygnus X-3 is the only one found so far), an d why the incidence of double neutron stars is so low. A better unders tanding of Common Envelope evolution is required in order to answer th ese questions. The role of velocity kicks imparted to neutron stars du ring supernova collapse is discussed. Such kicks might cause many runa way OB stars to be single.