Short of spectrophotometric studies - which require
extensive efforts and are currently unpractical and
unthinkable at major observatories -
intermediate-band photometry offers the best possibilities to
characterize the light variations of Ap stars.
Dedicated systems such as Maitzen's 
(Maitzen 1976)
are certainly ideal since they offer direct measurement
of physical parameters, but the Strömgren system has
proven to be quite valuable in many studies (see, e.g., Vogt & Faundez
1979; Mathys & Manfroid 1985). This is also
the case of the Geneva system which
combines wide and intermediate bands (see, e.g., North 1984; North
& Adelman 1995).
In this paper we present observations of 21 Ap stars, most of which were never studied for variations. About half of them show clear optical variations. We derived the periods -- not always unambiguously -- and determined analytical model of the light curves in 17 colours and indices.