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We present broad band (F583W, roughly V) photometry of two late M dwarf stars Proxima Centauri and Barnard's Star. These were secured with Fine Guidance Sensor 3 aboard HST . Our maximum temporal resolution is 25 ms during each observation. Intervals between observation sets average 10 days or longer. The Proxima Centauri data includes over one hundred 90 second segments secured over 2.2 years. We have monitored Barnard's Star for 1.5 years and collected around sixty segments. Results include episodes of low-amplitude long-term periodic variability in both stars. One of our observation sessions captured a major flare on Proxima Centauri. The star doubled in brightness within a few seconds in the F583W bandpass. We present an analysis to detect a position shift due to the flare. We detect no flare activity associated with Barnard's Star.
=46ine Guidance Sensor 3 is one of the most precise and is the highest-speed photometer on HST .
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