  We report the discovery of 8.5 sigma high-frequency quasi-periodic
oscillations (HFQPOs) at 66 Hz in the RXTE data of the black hole candidate IGR
J17091-3624, a system whose X-ray properties are very similar to those of
microquasar GRS 1915+105. The centroid frequency of the strongest peak is ~66
Hz, its quality factor above 5 and its rms is between 4 and 10%. We found a
possible additional peak at 164 Hz when selecting a subset of data; however, at
4.5 sigma level we consider this detection marginal. These QPOs have hard
spectrum and are stronger in observations performed between September and
October 2011, during which IGR J17091-3624 displayed for the first time light
curves which resemble those of the gamma variability class in GRS 1915+105. We
find that the 66 Hz QPO is also present in previous observations (4.5 sigma),
but only when averaging ~235 ksec of relatively high count rate data. The fact
that the HFQPOs frequency in IGR J17091-3624 matches surprisingly well that
seen in GRS 1915+105 raises questions on the mass scaling of QPOs frequency in
these two systems. We discuss some possible interpretations, however, they all
strongly depend on the distance and mass of IGR J17091-3624, both completely
unconstrained today.
