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We have carried out a timing analysis of Cyg X-1 and GRS 1915+105 using morethan 5 years worth of data from the All Sky Monitor (ASM) on board RXTE. Wehave obtained for the first time power density spectra, colour-colour andcolour-intensity plots, cross-correlation functions and phase-lag diagrams toinvestigate the variability of the sources at frequencies < 10^{-5} Hz. We findthat the PDS are not flat but consistent with a power-law of index ~-1. This isthe same slope as that found in the power spectra of Cyg X-1 during the softstate. In fact, the power spectrum of Cyg X-1 in the frequency range10^{-7}-10^{-5} Hz appears as a continuation of the 10^{-3}-10 Hz powerspectrum during the soft state. Significant variability, expressed as thefractional rms in the frequency range 2 x 10^{-7}-1 x 10^{-5} Hz, is detectedat a level of 27% and 21% for GRS 1915+105 and Cyg X-1, respectively. Ourresults confirm previous suggestions that the innermost regions of theaccretion disc are affected by variations occurring at larger radii from theblack hole, presumably due to changes in the mass accretion rate. We alsoobserve strong spectral changes: in Cyg X--1 the spectrum softens as the fluxincreases when it is in the low/hard state, and hardens with increasing flux inthe high/soft state. GRS 1915+105 follows the same trend as Cyg X-1 in the softstate, but the hardness ratios show a larger amplitude of variation. Thecross-correlation shows that the variations occur simultaneously in all energybands. However, the cross-correlation functions are asymmetric toward negativelags. The phase spectrum also reveals negative lags at periods larger than afew days, suggesting that the long term variations in the soft energy band aredelayed with respect to the harder bands.Comment: 8 pages, 9 figures, accepted for publication in A&

The aperiodic variability of Cyg X–1 and GRS 1915+105 at very low frequencies