Photometric monitoring of the ROSAT selected late type stars

We monitored the light variations of 16 solar-type stars recently discovered in the X-ray wave-length range during the ROSAT all-sky survey. We find that 9 out of 16 stars showed appreciable light variability with amplitudes of a few hundredths of a magnitude. They are all proved to be in periodic variations. Using the methods of the phase dispersion minimization (PDM) and Fourier Analysis (PERIOD04), we derive the photometric periods for these stars. The rotational periods are found range from 0.471 to 17.31 days and the period of stars most (of 7 stars) being shorter than 3 days. Apart from binaries system, the results give further evidence for the spin up of solar-type stars as predicted by models of angular momentum evolution of pre-main sequence stars.