The HIPPARCOS paylaod was optimised during its development to produce the highest achievable photometric accuracy. A complex calibration process allowed to correct rapid ageing effects due to cosmic irradiation. The time series accumulated during the 3.3 year mission led to the systematic detection of microvariability and variability among the 118200 programme stars, with very limited biases against particular periods or amplitudes. For 2700 stars periods and amplitudes could be precisely determined whereas for 5540 stars the light-curves were found non-periodic. The rate of new variables is high but dependent on the variability types. The whole HR-diagram could be descri- bed in terms of luminosity stability at the milimagnitude level. The subset of constant stars defined a new photometric reference system over the whole sky with an unprecedented accuracy and homogeneity. This system will play the role of a first epoch for the future detection of secular luminosity changes.
IAU Joint Discussion
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