LOW-AMPLITUDE VARIABILITY AND TRANSIENT PERIODICITY IN FF ANDROMEDAE AND OTHER ACTIVE STARS

This work presents the first results of a project mainly dedicated to the detection of low amplitude variability in stars with the URSULA do uble channel photometer fed by the 91 cm telescope of Catania Astrophy sical Observatory. URSULA simultaneously observes the target star and a nearby comparison star with two identical photometers and achieves a relative photometric precision down to a few thousandths of magnitude because rapid fluctuations of sky transparency equally affect both st ars and are easily removed. We can, therefore, use short integration t imes and attain high time resolution. The analysis of our observations is based on the methods of periodograms, according to the formulation for unequally spaced time sequences devised by Scargle (1982). We hav e observed FF And, BY Dra, V 1054 Oph and V 1396 Cyg and detected, at a high level of confidence, several cases of small scale variability, particularly evident in the data of FF And and of V1054 Oph. From FF A nd we have detected small, but highly significant, dips in the light c urve, approximately 25 minutes before a large flare. We compare this f inding with the known phenomenology of pre-flare dips and, among other interpretations, we propose them as possible signatures of pre-flare atmospheric activation. We also discuss the data set pertaining to all observed stars.