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Differential photometry of delta Scorpii during 2011 periastron

Hundred observations of Delta Scorpii over 200 days, from April 2 to October 16, 2011, have been made for AAVSO visually and digitally from Rio de Janeiro, Rome and Paris. The three most luminous pixels either of the target star and the two reference stars are used to evaluate the magnitude through differential photometry. The main sources of errors are outlined. The system of Delta Scorpii, a spectroscopic double star, has experienced a close periastron in July 2011 within the outer atmospheres of the two giant components. The whole luminosity of Delta Scorpii system increased from about Mv=1.8 to 1.65 peaking around 5 to 15 July 2011, but there are significant rapid fluctuations of 0.2 - 0.3 magnitudes occurring in 20 days that seem to be real, rather than a consequence of systematic errors due to the changes of reference stars and observing conditions. This method is promising for being applied to other bright variable stars like Betelgeuse and Antares. After August the magnitude remained constant at Mv=1.8 until the last observation on October 16 made in twilight from Rome.