All Figures

	Figure 1: Time series fields in the  Ori cluster: the TLS field is indicated with solid, the CA field with dashed lines. The positions of the cluster member candidates (see Table 7) are overplotted.  Ori is the bright star near the bottom of the field.
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In the text

Figure 2: (I,I-J) colour magnitude diagram for the CA ( upper panel) and the TLS field ( lower panel). All probable cluster members are marked as larger dots. Error bars indicate typical photometry errors for the candidates. The separation line between cluster member candidates and field objects is shown as a straight line. The position of the 3 Myr isochrone from Baraffe et al. (1998) is indicated by the right-hand line. The arrow shows the reddening vector for  mag.

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In the text

Figure 3: (J,J-H) colour magnitude diagram for all candidates (plotted as filled squares). Error bars indicate typical photometry errors for the candidates. The 3 Myr isochrone for  Ori (Baraffe et al. 1998) is shown as a solid line. For bright objects, the isochrones for 1 Myr (dotted line) and 5 Myr (dashed line) differ significantly from the 3 Myr track and are therefore also shown. Objects whose J-H colour deviates clearly from the cluster isochrone are marked with crosses and excluded. The positions of 1000 arbitrarily selected field stars are shown as small dots.

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In the text

	Figure 4: Datapoint distribution for the TLS ( upper panel) and the CA run ( lower panel). Plotted is the non-integer fraction of the observing times against the observing times.
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In the text

Figure 5: Root mean square of the time series photometry vs. I-band magnitude for all CA candidates. The solid line is the mean rms of the relative photometry, determined by fitting the rms of all objects in the field. Below I=14 mag the long exposure photometry is influenced by saturation, brighter targets were analysed with the short exposure lightcurves. All objects marked with a cross are variable at the 99% level.

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In the text

	Figure 6: Root mean square of the time series photometry vs. I-band magnitude for all TLS candidates. The solid line is the mean rms of the relative photometry, determined by fitting the rms of all objects in the field. All objects marked with a cross are variable at the 99% level. At the bright end, the variability test is contaminated by extinction residuals and beginning saturation.
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In the text

	Figure 7: Phased lightcurves for the detected periodicities. No. and period from Tables 3 and 4 are indicated.
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In the text

	Figure 8: Phased lightcurves for the detected periodicities (continued).
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In the text

	Figure 9: Sensitivity of the period search: the absolute difference between detected period and true period vs. true period for the TLS ( upper panel) and the CA run ( lower panel). The signal-to-noise ratio of the shown periodicity is 5; the dotted line corresponds to a period error of 10%. The vertical bars indicate the periods found with our time series analysis.
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In the text

	Figure 10: Pooled variance diagrams for selected lightcurves. The solid lines show a running median over five datapoints. The periods adopted in Tables 3 and 4 are indicated as dashed line.
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In the text

	Figure 11: Angular velocity versus amplitude. The dashed line delineates the separation between low-amplitude and high-amplitude objects.
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In the text

Figure 12: (H-K, J-K) colour-colour diagram for the cluster member candidates in our field. Crosses are periodically variable objects. Overplotted symbols flag objects with high photometric amplitude: squares mean that the lightcurve amplitude exceeds 0.25 mag, and thus mark the objects defined as high-amplitude targets in Sect. 5. Triangles are assigned to transition objects with lightcurve amplitudes between 0.1 and 0.25 mag. The solid line indicates the 3 Myr isochrone from the Baraffe et al. evolutionary tracks. Dotted lines show the interstellar extinction vector calculated from Mathis (1990). A reddening vector for  mag is given. For some reddened objects, we overplot the errorbars for the photometry.

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In the text

Figure 13: Mass dependence of the rotation period: plotted are all measured periods from TLS and CA vs. the object mass. The solid line shows the median period published by Herbst et al. (2001). The median for our objects with is shown as filled square, vertical lines indicate the quartiles (giving an estimate of the statistical uncertainties). This median found for our objects extends the line of Herbst et al. (2001) into the Brown Dwarf regime (dashed line).

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In the text

	Figure 14: Optical spectra for three highly variable targets (from top to bottom: Nos. 2, 33, 43) and one non-variable target (No. 90, beneath). The spectra are shifted by 12, 6, 4, 0 (from top to bottom) units for clarity.
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In the text