of 1 to 2 minutes. There are differences in the observations presented by these authors, and for this reason we separate them in two classes. The first, class $\rho$ (Fig. 2 q,r), is extremely regular in the light curve, and in the CD it presents a loop-like behavior (described as ring-like in [@vil98], where data with lower time resolution were considered).

$\bullet$ class $\nu$

:   There are two main differences between observations in this class and those of class $\rho$. The first is that they are considerably more irregular in the light curve, and at times they show a long quiet interval, where the source moves to the right part of the CD (see Fig. 2s,t). The second is that, at 1s time resolution, they show more structure in the profile of the ‘flares’, notably a secondary peak after the main one (see Fig. 17b).

$\bullet$ class $\alpha$

:   Light curves of observation intervals of class $\alpha$ show long ($\sim$1000 s) quiet periods, where the count rate is below 10000 cts/s, followed by a strong ($>$20000 cts/s) flare and a few 100s of seconds of oscillations(see Fig. 2u,v). The oscillations start at a time scale of a few dozen seconds 