  Irradiance variability has been monitored from space for more than two
decades. Even if data are coming from different sources, it is well established
that a temporal variability exists which can be set to as approximately 0.1%,
in phase with the solar cycle. Today, one of the best explanation for such an
irradiance variability is provided by the evolution of the solar surface
magnetic fields. But if some 90 to 95% can be reproduced, what would be the
origin of the 10 to 5% left? Non magnetic effects are conceivable. In this
paper we will consider temporal variations of the diameter of the Sun as a
possible contributor for the remaining part. Such an approach imposes strong
constraints on the solar radius variability. We will show that over a solar
cycle, variations of no more than 20 mas of amplitude can be considered. Such a
variability (far from what is reported by observers conducting measurements by
means of ground-based solar astrolabes) may explain a little part of the
irradiance changes not explained by magnetic features. Further requirements are
needed that may help to reach a conclusion. Dedicated space missions are
necessary (for example PICARD, GOLF-NG or SDO, scheduled for a launch around
2008); it is also proposed to reactivate SDS flights for such a purpose.
