A coupling means (10) provided with a first member (20) suitable for being fastened to a first rotary part (2) and with a second member (30) suitable for being fastened to a second rotary part (3), the first member (20) being provided with a first diaphragm (22) and the second member (30) being provided with a second diaphragm (32) that is secured to the first diaphragm (22). An emergency torque transmission device comprises at least one bayonet system including a protuberance (50) secured to one member (30) and co-operating with an angled groove (60) secured to the other member (20) by being inserted in the angled groove (60) by a thrust-and-rotation movement, in the absence of said breakage said coupling means (10) including both axial clearances in compression (70′) and in translation (70″) and also circumferential clearance between each protuberance (50) and walls defining the corresponding angled groove (60).