left along the direction of the elongation (See Fig. 2e).

$\bullet$ class $\mu$

:   These are observations that, although showing large amplitude (more than a factor of two) variability, show a single ‘branch’ of points in the CD (class $\mu$, Fig. 2g,h). This branch is elongated diagonally and it is curved to the right in its lower part. The light curves show rapid quasi-periodic oscillations (typical time scale between 10 seconds and 100 seconds), sometimes alternated with stabler periods of $\sim$100 seconds duration.

$\bullet$ class $\delta$

:   A number of observations show red-noise-like variability, which can exceed a factor of two in range. These observations form class $\delta$. Notice that in a number of observations in this class, there are ‘dips’ in the light curve which, unlike the rest of the variability, are characterized by a significant softening in both colors (see Fig. 2i,j). These dips last typically 10-20 seconds and, while the out-of-dip count rate is always above 10000 cts/s, during the dip it decreases to 10000 cts/s or less.

$\bullet$ class $\theta$

:   A group of observations clearly distinguishable from the others is class $\theta$ (Fig. 2k,l): the light curve has a characteristic shape, with ‘M’ shaped intervals 