  We present a detailed study of the 5 Hz quasi-periodic oscillation (QPO)
recently discovered in the bright X-ray transient and black-hole candidate GRS
1739-278 (Borozdin & Trudolyubov 2000) during a Rossi X-ray Timing Explorer
observation taken on 1996 March 31. In total 6.6 ksec of on-source data were
obtained, divided in two data sets of 3.4 and 3.2 ksec which were separated by
2.6 ksec. The 5 Hz QPO was only present during the second data set. The QPO
increased in strength from below 2% rms amplitude for photon energies below 4
keV to ~5% rms amplitude for energies above 10 keV. The soft QPO photons (below
5 keV) lagged the hard ones (above 10 keV) by almost 1.5 radian. Besides the
QPO fundamental, its first overtone was detected. The strength of the overtone
increased with photon energy (from <2% rms below 5 keV to ~8% rms above 10
keV). Although the limited statistics did not allow for an accurate
determination of the lags of the first overtone, indications are that also for
this QPO the soft photons lagged the hard ones. When the 5 Hz QPO was not
detected (i.e., during the first part of the observation), a broad noise
component was found for photon energies below 10 keV but it became almost a
true QPO (with a Q value of ~1.9) above that energy, with a frequency of ~3 Hz.
Its hard photons preceded the soft ones in a way reminiscent of the 5 Hz QPO,
strongly suggesting that both features are physically related. We discuss our
finding in the frame work of low-frequency QPOs and their properties in BHCs.
