large amplitude variations (less than a factor of two) nor obvious structured variability (only random noise is seen in light curves binned at 1 second). However, examination of their light curves and CDs shows that these can be further subdivided into two separate classes: The first, class $\phi$, is characterized by a count rate of $\sim$10 kcts/s or less and by a particular position in the CD, namely HR$_1\sim$1, HR$_2\sim$0.05 (see Fig. 2a,b).

$\bullet$ class $\chi$

:   The second, class $\chi$, has a more varied count rate, ranging between 3400 and $\sim$30000 cts/s (for 5 PCU units) and it occupies a position in the CD which can vary but is always at HR$_2>$0.1 (see Fig. 2c,d). The cloud of points in the CD is usually diagonally elongated.

$\bullet$ class $\gamma$

:   There are observations that are relatively quiet (class $\gamma$), apart from quasi-periodic oscillations with a typical time scale of $\sim$10 s and/or the presence of sharp ’dips’ with a typical duration of a couple of seconds (see Fig. 2f). In the CD, the quasi-periodicity results in a diagonally elongated distribution of points. The points corresponding to the dips are separated from the main distribution, and are located on its 