  The two high frequency quasi periodic oscillations (HFQPOs) recently reported
in the black hole candidate IGR J17091-3624 by Altamirano and Belloni (2012)
are in a 5:2 frequency ratio (164 Hz to 66 Hz). This ratio is strongly
suggestive of period doubling and nonlinear resonance analogous to phenomena
known in RV Tauri-type pulsating stars (and recently discovered also in
oscillations of RR Lyrae-type and of BL Herculis-type variables). An
interpretation of the frequency ratio in terms of nonlinear interactions and a
comparison with the HFQPOs reported in GRS 1915+105 may imply a mass of about 6
solar masses for the black hole in IGR J17091- 3624.
