As part of the NASA Kepler Guest Observer program , we requested and obtained long-cadence data on about 2700 faint ( Kepler magnitude 14-16 ) stars with effective temperatures and surface gravities that lie near or within the pulsation instability region for main-sequence \gamma Doradus and \delta Scuti pulsating variables .
These variables are of spectral type A-F with masses of 1.4 to 2.5 solar masses .
The \delta Scuti stars pulsate in radial and non-radial acoustic modes , with periods of a few hours ( frequencies \sim 10 cycles/day ) , while \gamma Doradus variables pulsate in nonradial gravity modes with periods 0.3 to 3 days ( frequencies \sim 1 cycle/day ) .
Here we consider the light curves and Fourier transforms of 633 stars in an unbiased sample observed by Kepler in Quarters 6-13 ( June 2010-June 2012 ) .
We show the location of these stars in the log surface gravity–effective temperature diagram compared to the instability region limits established from ground-based observations , and taking into account uncertainties and biases in the Kepler Input Catalog T _ { eff } values .
While hundreds of variables have been discovered in the Kepler data , about 60 % of the stars in our sample do not show any frequencies between 0.2 and 24.4 cycles/day with amplitude above 20 parts per million .
We find that six of these apparently constant stars lie within the pulsation instability region .
We discuss some possible reasons that these stars do not show photometric variability in the Kepler data .
We also comment on the ‘ non-constant ’ stars , and on 26 variable-star candidates , many of which also do not lie within the expected instability regions .