7 Hydrodynamic Instabilities in Close Binary Systems (Frederic A. Rasio) 121 7. 1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . 121 7. 1. 1 The stability of self-gravitating fluid equilibria 121 7. 1. 2 Astrophysical motivation . 123 7. 1. 3 Common envelope systems 125 7. 2 Dynamical instabilities. . . . . . . 126 7. 2. 1 Physical mechanism . . . . 126 7. 2. 2 Application to coalescing neutron star binaries 127 7. 3 Secular instabilities. . . . . . . . . . . 130 7. 3. 1 Physical mechanism . . . . . . 130 7. 3. 2 Application to contact binaries 133 8 Common Envelope Evolution in Binary Systems (Mario Livio) 141 8. 1 Introduction. . . . . . . . . . . . . . . . . . . . 141 8. 2 The entrance into the common envelope phase . . . . . 142 8. 3 The outcome of the CE phase. . . . . . . . . . . . . . . 145 8. 4 How close can we get to observing the common envelope Phase? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 8. 4. 1 How can PNe with binary nuclei be used to constrain CE physics . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 8. 4. 2 How can nova systems be used to constrain CE physics 148 8. 4. 3 Other tests of common envelope evolution 150 8. 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . 151 9 Structure and Evolution of Massive Close Binaries (Dany Vanbeveren) 155 9. 1 Introduction. . . . . . . . . . . . . . . . . . 155 9. 2 Definitions. . . . . . . . . . . . . . . . . . . 156 9. 3 Intermediate mass and massive single stars 156 9. 3. 1 Observations . . . . . . . . . . . . . 156 9. 3. 2 Stellar structure equations for non-rotating IMS's and MS's 160 9. 3. 3 Evolutionary computations of non-rotating IMS's and MS's 162 9. 3. 4 Overall comparison with observations '" 163 9. 3. 5 The role of rotation in single star evolution . . .