A recent photometric survey in the NGC 3766 cluster led to the detection of stars presenting an unexpected variability .
They lie in a region of the Hertzsprung-Russell ( HR ) diagram where no pulsation are theoretically expected , in between the \delta Scuti and slowly pulsating B ( SPB ) star instability domains .
Their variability periods , between \sim 0.1–0.7 d , are outside the expected domains of these well-known pulsators .
The NCG 3766 cluster is known to host fast rotating stars .
Rotation can significantly affect the pulsation properties of stars and alter their apparent luminosity through gravity darkening .
Therefore we inspect if the new variable stars could correspond to fast rotating SPB stars .
We carry out instability and visibility analysis of SPB pulsation modes within the frame of the traditional approximation .
The effects of gravity darkening on typical SPB models are next studied .
We find that at the red border of the SPB instability strip , prograde sectoral ( PS ) modes are preferentially excited , with periods shifted in the 0.2–0.5 d range due to the Coriolis effect .
These modes are best seen when the star is seen equator-on .
For such inclinations , low-mass SPB models can appear fainter due to gravity darkening and as if they were located between the \delta Scuti and SPB instability strips .