  We studied the properties of the low-frequency quasi-periodic oscillations
detected in a sample of six black hole candidates (XTE J1550-564, H 1743-322,
XTE J1859+226, 4U 1630-47,GX 339-4, XTE J1650-500) observed by the Rossi XTE
satellite. We analyzed the relation between the full width half maximum and the
frequency of all the narrow peaks detected in power density spectra where a
type-C QPO is observed. Our goal was to understand the nature of the modulation
of the signal by comparing the properties of different harmonic peaks in the
power density spectrum. We find that for the sources in our sample the width of
the fundamental and of the first harmonic are compatible with a frequency
modulation, while that of the sub-harmonic is independent of frequency,
possibly indicating the presence of an additional modulation in amplitude. We
compare our results with those obtained earlier from GRS 1915+105 and XTE
J1550-564.
