  The Optical Monitoring Camera (OMC) onboard INTEGRAL provides photometry in
the Johnson V-band. With an aperture of 50 mm and a field of view of 5deg x
5deg, OMC is able to detect optical sources brighter than V~18, from a
previously selected list of potential targets of interest. After more than nine
years of observations, the OMC database contains light curves for more than
70000 sources (with more than 50 photometric points each). The objectives of
this work have been to characterize the potential variability of the objects
monitored by OMC, to identify periodic sources and to compute their periods,
taking advantage of the stability and long monitoring time of the OMC. To
detect potential variability, we have performed a chi-squared test, finding
5263 variable sources out of an initial sample of 6071 objects with good
photometric quality and more than 300 data points each. We have studied the
periodicity of these sources using a method based on the phase dispersion
minimization technique, optimized to handle light curves with very different
shapes.In this first catalogue of variable sources observed by OMC, we provide
for each object the median of the visual magnitude, the magnitude at maximum
and minimum brightness in the light curve during the window of observations,
the period, when found, as well as the complete intrinsic and period-folded
light curves, together with some additional ancillary data.

Instrumentation and Methods for Astrophysics;Solar and Stellar Astrophysics