The domain of gamma Doradus variables in the Hertzsprung-Russell diagram

70 new gamma Doradus candidates were identified from Hipparcos photometry, which represents more than a doubling of the presently known number. Select ing the objects with good evidence for multiperiodicity, it is found that t hese stars, together with bona fide members of the class, occupy a well-def ined region in a colour-magnitude diagram. This domain corresponds to a ran ge of 7200-7700 K on the zero-age main sequence (ZAMS) and 6900-7500 K one magnitude above it, which partly overlaps with the instability strip of del ta Scuti stars. For the first time, gamma Doradus stars can be discussed as a group. They c an be found over a significant fraction of the main sequence lifetimes for objects in the relevant temperature range. An upper Limit on the surface me tallicity of gamma Doradus stars is apparent, which may guide the search fo r their pulsation driving mechanism. The importance of possible objects exh ibiting both gamma Doradus and delta Scuti-type pulsations is discussed.