On April 28 1986 Cornelis de Jager reached the age of 65 years. On April 30 he officially retired from the University of Utrecht where he has held a Chair for Stellar Astrophysics, later changed into Space Physics, since 1958. Cees de Jager, as he prefers to be called by his friends, has had an active and successful life in science. His interest in astronomy was raised by his father under the clear skies of Celebes (Indonesia). He started a study in physics and astronomy as a student of the late M. Minnaert in Utrecht during World War II. When in 1943 the occupying forces recruited students who did not want to sign the declaration of loyalty for their war-efforts, Cees and three fellow students went into hiding at the Observatory in Utrecht. During this very "quiet" period van de Hulst developed the theory of the 21 em radiation of neutral hydrogen and de Jager started the observations of variable stars in the pitch dark nights of a country at war. The study of Beta Cepheids rapid­ ly awoke his interest which was kept throughout the years. In 1958 he organized an international campaign to observe 12 Lac spectroscopically and photometrically, which was a great success.

Section I: Invited Reviews and General Discussions.- The upper HR diagram - An observational overview.- Observations and interpretations of stellar pulsations.- Theory of non-radial pulsations in massive early-type stars.- Theory of vibrational instabilities in luminous early type stars.- Observed variations in O and Of stars.- Observed variations in Wolf-Rayet stars.- Variations in luminous blue variables.- Giant outbursts of the Eta Carinae - P Cygni type.- Instabilities due to convection and rotation.- Shock waves in luminous early-type stars.- Radiation driven instabilities.- Stellar instabilities in the upper part of the Hertzsprung-Russell Diagram.- Section II: Poster Papers.- Massive stars in nearby galaxies.- Rate of mass-loss in the Hertzsprung-Russell Diagram.- The unstable of 06.5f?p star HD 148937 and its interstellar environment.- Two comments of the Beta Cephei variable 12 Lacertae.- Non radially pulsating Wolf-Rayet stars.- Sobolev type line profile in case of non radial wind density perturbations.- Episodic distortion and dust formation in the wind of WR140.- Photometric variability of Wolf-Rayet stars.- Improved mass loss rates for WC7-9 stars: their effect on the Wolf-Rayet stellar wind momentum problem.- Variability of Wolf-Rayet stars in linear polarization.- An extended nebulosity surrounding the S Dor variable R127.- The LMC-S Dor variable R 71 - an IRAS point source.- Eclipse spectrum of the LMC P Cyg star R 81.- The light- and colour variation of Eta Carinae for the years 1983-1986 in the VBLUW system.- Do superluminous stars really explode?.- Radiation pressure in acoustic wave calculations of early type stars.- The influence of photospheric turbulence on stellar mass loss.- Nonlinear dynamics of instabilities in line-driven stellar winds.-The X-ray emission of Tau Sco, Bo V, and the problems posed for embedded shock models.- Name index.- Object index.