On the relationship between Of and WR stars.

Current knowledge about the physical properties of Of, WR, and P Cygni stars is reviewed. Criteria for each classification are outlined, and it is noted that all three types are characterized by a stellar wind and a circumstellar envelope. Some sample spectra of Of, WR, and P Cyg stars are presented, WR stars are shown to have more extensive envelopes than Of stars, and the 'peeled onion skin' model of WR stars is described. Mass-loss rates for P Cyg stars are discussed, techniques for deriving the parameters of Of and WR stars are summarized, and an H-R diagram is plotted for O, Of, and WR stars. A unifying hypothesis relating O, Of, and WR stars is proposed according to which O stars become Of stars if they are sufficiently massive and luminous for a substantial wind to be generated, the mass-loss rate in single Of stars may be sufficient to produce single WR stars, and the mass-loss rate in binary Of stars must be sufficient to produce binary WR stars.