Astronomy Astrophysics manuscript no.

We have analyzed the X-ray variability of dF7-dK2 stars in the solar neighborhood detected with the pointed ROSAT-PSPC observations. Our data base is the sample of all stars listed in the CNS3 catalog (Gliese & Jahrei 1991) having a B-V color between 0.5 and 0.9; it includes 70 pointed observations of 40 distinct stars or multiple systems. We have applied the unbinned Kolmogorov-Smirnov test on all X-ray photon time series of our sample: only 10 observations relative to 8 distinct stars are variable at a con dence level greater than 99% and 4 of them belong to multiple systems. For the subsample of 9 stars observed both at the beginning and at the end of the mission, we can study the variability on time scale of years and compare amplitude variations at short and long time scales. Our analysis suggests that, for these stars, the X-ray variability is more likely on longer time scale. All the stars variable on long time scale, and not on short time scale, are relatively quiet and similar to the Sun, suggesting that the variations may be due to cycles. The comparison of our results with those previously obtained for dM stars shows that the amplitude of variability of X-ray emission from dF7-dK2 stars is smaller than that observed in dM stars