  We have carried out a timing analysis of Cyg X-1 and GRS 1915+105 using more
than 5 years worth of data from the All Sky Monitor (ASM) on board RXTE. We
have obtained for the first time power density spectra, colour-colour and
colour-intensity plots, cross-correlation functions and phase-lag diagrams to
investigate the variability of the sources at frequencies < 10^{-5} Hz. We find
that the PDS are not flat but consistent with a power-law of index ~-1. This is
the same slope as that found in the power spectra of Cyg X-1 during the soft
state. In fact, the power spectrum of Cyg X-1 in the frequency range
10^{-7}-10^{-5} Hz appears as a continuation of the 10^{-3}-10 Hz power
spectrum during the soft state. Significant variability, expressed as the
fractional rms in the frequency range 2 x 10^{-7}-1 x 10^{-5} Hz, is detected
at a level of 27% and 21% for GRS 1915+105 and Cyg X-1, respectively. Our
results confirm previous suggestions that the innermost regions of the
accretion disc are affected by variations occurring at larger radii from the
black hole, presumably due to changes in the mass accretion rate. We also
observe strong spectral changes: in Cyg X--1 the spectrum softens as the flux
increases when it is in the low/hard state, and hardens with increasing flux in
the high/soft state. GRS 1915+105 follows the same trend as Cyg X-1 in the soft
state, but the hardness ratios show a larger amplitude of variation. The
cross-correlation shows that the variations occur simultaneously in all energy
bands. However, the cross-correlation functions are asymmetric toward negative
lags. The phase spectrum also reveals negative lags at periods larger than a
few days, suggesting that the long term variations in the soft energy band are
delayed with respect to the harder bands.
