(FORTUNE Magazine) – As the two Germanies unite, an American with a lot to sing about is General Motors' Louis R. Hughes, 41, chairman of the company's Adam Opel AG subsidiary in Russelsheim, West Germany. His home marketplace expands some 26% overnight. Says Hughes: ''This is the opportunity of the century. We think East Germany is going to grow the fastest of any East European market.'' Production for Opel's Vectra starts this October at a 10,000-car-a-year plant in Eisenach, East Germany. The intermediate-size car will be sold throughout Europe. GM plans another East German plant, which could produce 150,000 vehicles a year. Other GM ventures include factories for engines in Hungary and transmissions in Czechoslovakia. Hughes, a Cleveland native, keen mountain climber, and career-long GM man, took over Opel last year after two years as chief financial officer for GM Europe. Since then, Opel's West German market share has climbed from 16% to 17%, and 1990 profits are expected to rise 27% to about $900 million. Contributing to Opel's success: the sporty Calibra coupe, which is sold out through 1992. Hughes's fans include Wolfram Liedtke, 45, head of East Germany's Automobilwerk Eisenach, Opel's partner in the new joint venture. Says he: ''The man is extraordinarily clever. He has a good nose for the development of an Eastern European business strategy.'' Do one Germany and a united Europe, not to mention business possibilities in the Soviet Union, keep Hughes awake at night? Not at all. ''Increasing market share, a vastly improved image, factories operating full blast, and the Calibra sold out,'' he says, ''make it easy to sleep.''