Ravinia's inaugural "Jazz in June" series has received kudos for booking Oscar Peterson (who plays Saturday night) but hardly a nod for also booking the only pianist who might be able to outplay him. Then again, that's the story of Dorothy Donegan's career. A combination of factors keep her underrecognized: When she broke into jazz in the early 40s (bringing with her a double fistful of classical technique from Chicago's American Conservatory of Music), women instrumentalists were an even rarer breed than they are today. In addition, Donegan has always been as much entertainer as musician, and her free-associative musical antics--combined with her willingness to interrupt the music for a bit of wild-eyed banter--has put off lots of critics. But like the highly skilled acrobats who become circus clowns, she couldn't fool around with this stuff if she wasn't so good at it; in fact, her sheer virtuosity and careening, multireferenced arrangements place her at a level occupied by only a handful of this century's pianists (Art Tatum, Earl Hines, Peterson, and one or two others). And at 69 she's still a hoot and a half. Also appearing are the Harper Brothers, six musicians less than half Donegan's age who share her conviction that serious music can be fun. (Potential Conflict of Interest Warning: I wrote the program notes for the Ravinia series. But I've loved Donegan for years.) Wednesday, 8 PM, Murray Theatre, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 728-4642.