Fast photon-counting optical astronomy

. Fast sub-second optical variability exists in some important classes of astrophysical sources, in particular pulsars and X-ray binaries. We regularly perform ultra-fast optical photometric campaigns on some of the most interesting targets and transient events. At the same time, we are tackling new campaigns searching for fast optical variability in other types of sources (Fast Radio Bursts, magnetars). To this end we use and develop dedicated fast photon counting instrumentation in the visible band, an area in which our team is the leader. Observations are carried out within the framework of multiwave-length campaigns and in synergy with facilities operating at other wavelengths. Here we report on some of the most recent observing campaigns and technological developments, and on the outcome of a km-baseline experiment of stellar intensity interferometry carried out in photon counting mode.