Dear Gianluca, Thank you very much for your astrometry! > Unfortunately, clouds prevent me from making time series photometry. Please continue, as long as possible, your photometry, even in the presence of intervening clouds (never give up as long as the brighter stars in the frame are visible). In the case of very large extinction by clouds, lengthen the exposure time (can be as long as minutes). Such frames, even affected by clouds, are often measurable, often against the observer's impression, using apparopriate PSF photometry software. Short exposures in the presense of passing clouds are often hazardous, but if you have occasionally passing clear gaps between clouds, it will be useful to continue short exposures. Some "tips" from the ever-cloudy country... Regards, Taichi Kato