  We present a systematic analysis of the phase lags associated with the type-C
QPOs in GRS 1915+105 using RXTE data. Our sample comprises of 620 RXTE
observations with type-C QPOs ranging from ~0.4 Hz to ~6.3 Hz. Based on our
analysis, we confirm that the QPO phase lags decrease with QPO frequency, and
change sign from positive to negative at a QPO frequency of ~2 Hz. In addition,
we find that the slope of this relation is significantly different between QPOs
below and above 2 Hz. The relation between the QPO lags and QPO rms can be well
fitted with a broken line: as the QPO lags go from negative to positive, the
QPO rms first increases, reaching its maximum at around zero lag, and then
decreases. The phase-lag behaviour of the subharmonic of the QPO is similar to
that of the QPO fundamental, where the subharmonic lags decrease with
subharmonic frequency and change sign from positive to negative at a
subharmonic frequency of ~1 Hz; on the contrary, the second harmonic of the QPO
shows a quite different phase-lag behaviour, where all the second harmonics
show hard lags that remain more or less constant. For both the QPO and its
(sub)harmonics, the slope of the lag-energy spectra shows a similar evolution
with frequency as the average phase lags. This suggests that the lag-energy
spectra drives the average phase lags. We discuss the possibility for the
change in lag sign, and the physical origin of the QPO lags.
