receiving the conduit and radial seal is obtained between the conduit and the coupling without necessitating a great tightening force on the o ring for easily releasable tightening after joining use can be made of a rider 5 the legs 7 of which engage behind the inner bead 2 and in a groove 6 formed in the outside of the coupling the front bead 2 u2032 may consist of a widened end portion of the conduit 1 it will be appreciated that no coupling is required for connecting 4 two conduits according to fig3 the second conduit 1 u2032 can by pressure turning be provided with a widened end portion 8 for receiving the end of the conduit 1 which is provided with the beads 2 2 u2032 and the o ring 3 and with for instance a posteriorly situated bead 8 u2032 for forming the groove 6 for a rider leg 7 as described above furthermore any suitable means other than a rider as above can be used for tightening e g a self threading screw 9 which is screwed into a bore 4 u2032 a formed in e g the wall of the coupling 4 u2032 as shown in fig4 the screw being substantially parallel with the connecting direction the underside of the screw head or a washer being supported against the rear bead 2 according to another preferred embodiment see fig5 the two tubes are kept together by a u shaped rider 10 the legs of the rider extend through holes 12 12 u2032 in the outer tube such as the conduit 1 u2032 or any of the receivers 4 4 u2032 the holes are axially.