in X-ray channels can be caused partially by the stochastic variations in the total intensity of the emission from the sources, which were simultaneously within the instrument’s field of view. Both, Prognoz 9 and “Mir” GRIF experiments were capable of eliminating the effect of some background factors with the use of direct measurements of the individual background components. Significant sporadic rises in the count rates in the X-ray channels were mainly attributed to hard X-ray bursts from solar flares and cosmic gamma-ray bursts. Since the latter were recorded during the experiments rather rarely [@Kud88; @Kud02], they could not significantly affect the background in the search for periodicities. In the Prognoz 9 experiment the Sun was constantly within instrument’s field of view, thus its X-ray activity was monitored continuously, and about 800 solar X-ray bursts were detected [@Abr88]. To identify such bursts the data from the Prognoz 9 RF experiment [@Val79], during which the Sun was continuously monitored in the band 2-10 keV by an instrument with a small ($<10^o$) field of view, as well as the data of ground-based optical and radio observations of the Sun [@Cof84] were used. In the search for periodicities the time intervals during which the 