gamma Doradus and delta Scuti stars: Cousins or twins?

gamma Doradus stars are a group of slowly pulsating early F dwarfs with typical low frequencies near 1 c/d. The pulsation modes are probably g modes. These stars are slightly cooler than the typical delta Scuti variables, which pulsate with higher frequencies of typically 10 c/d (mostly nonradial p modes). This paper examines the relationship between the two groups of variables. Additional low-frequency variability has been reported for fewer than 10% of the 300 delta Scuti stars examined. In at least one case (1 Mon), the observational data are consistent with an alternate interpretation in terms of a mode interaction, f(1) - f(2), between two p modes of higher frequency, rather than by the excitation of g modes. In several other stars (e.g. 4 CVn), the reported low-frequency variability may originate in the comparison star used. Arguments are given that for a number of stars the observed low-frequency variations are caused by observational errors (such as instrumental drift and transparency variations). A few delta Scuti stars remain for which intrinsic gamma Doradus-type variability may already have been discovered, although no case is well-studied. One of the best candidates is the star BI CMi, which is situated in the small region of overlap of the two groups of pulsators in the Hertzsprung-Russell diagram. The star deserves further study.