On the relationship between the δ Scuti and γ Doradus pulsators

We searched for delta Scuti-type pulsations amongst known and candidate gamma Doradus stars. The motivations for such a project come from the need to understand the relationship of these two classes of pulsator better, from the present poor knowledge of the hot border of the gamma Doradus phenomenon, and from the exciting prospects for asteroseismology should stars be found which have both types of pulsation excited. We acquired 270 h of observations and monitored a total of 26 stars. One target, HD 209295, turned out to be a member of both classes of pulsating star, but this object is peculiar in the sense that it is a close binary. We classify six of our targets as new bona fide gamma Doradus stars, whereas nine more are good gamma Doradus candidates, and three turned out to be ellipsoidal variables. One of our programme stars was found to be a delta Scuti star, with no additional gamma Doradus variations. Furthermore, one star was already known to be a bona fide gamma Doradus star, and we could not find an unambiguous explanation for the variability of five more stars. The analysis of our data together with improved knowledge of stars from the literature enabled us to revise the blue border of the gamma Doradus phenomenon towards cooler temperatures. This new blue edge is much better defined than the previous one and extends from a temperature of about 7550 K on the ZAMS to 7400 K one magnitude above it. Five bona fide gamma Doradus stars we observed are located inside the delta Scuti instability strip, but none of them exhibited observable delta Scuti pulsations. We therefore suggest that gamma Doradus stars are less likely to be delta Scuti pulsators compared with other normal stars in the same region of the lower instability strip. In addition, we show that there is a clear separation between the pulsation constants Q of delta Scuti and gamma Doradus stars. The gamma Doradus stars known to date all have Q > 0.23 d.