Preparing the COROT space mission:: Incidence and characterisation of pulsation in the lower instability strip

By pursuing the goal to find new variables in the COROT field-of-view we characterised a sample of stars located in the lower part of the instability strip. Our sample is composed of stars belonging to the disk population in the solar neighbourhood. We found that 23% of the stars display multiperiodic light variability up to a few mmag in amplitude, i.e., easily detectable on a single night of photometry. uvbybeta photometry fixed most of the variables in the middle of the instability strip and high-resolution spectroscopy established that they have v sin i > 100 km s(-1). An analysis of the Rodriguez & Breger ( 2001) sample (delta Sct stars in the whole Galaxy) shows slightly different features, i. e., most delta Sct stars have a 0.05-mag redder (b-y)(0) index and lower v sin i values. Additional investigation in the open cluster NGC 6633 confirms the same incidence of variability, i.e., around 20%. The wide variety of pulsational behaviours of delta Sct stars ( including unusual objects such as a variable beyond the blue edge or a rapidly rotating high-amplitude pulsator) makes them very powerful asteroseismic tools to be used by COROT. Being quite common among bright stars, delta Sct stars are suitable targets for optical observations from space.