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INTRODUCTION
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GRS 1915+105 was discovered in 1992 with WATCH (Castro-Tirado, Brandt & Lund 1992) as a transient source with considerable variability (Castro-Tirado et al. 1994). It was the first Galactic object to show superluminal expansion in radio observations (Mirabel & Rodríguez 1994). The standard interpretation of this phenomenon in terms of relativistic jets (Rees 1996) placed the source at a distance D=12.5 kpc, with a jet axis at an angle $i$=70$^\circ$ to the line of sight (Mirabel & Rodríguez 1994, Rodríguez & Mirabel 1999, but see Fender et al. 1999). A counterpart has been found in the infrared (Mirabel et al. 1994), but the high Galactic extinction prevents the detection of an optical counterpart. Before the launch of the Rossi X-ray Timing Explorer (RXTE), not many observations of this source existed in X rays: the WATCH and SIGMA instruments on board GRANAT and BATSE/GRO showed once again that it was very variable in the X-ray band (Sazonov et al. 1994, Paciesas et al. 1996).

Since the launch of RXTE, the source has been monitored continuously with the All-Sky Monitor (ASM) instrument, which showed that the 2-10 keV X-ray flux of the source is 