different scenarios and to compare them with the dependence (temperature and order) on the magnetic field reported above.

In conclusion, we have shown that in both powder and single crystal samples, a first-order magnetic transition occurs in [Yb$_2$Ti$_2$O$_7$]{}. The transition width is about 20 mK while the hysteresis at the transition is about 3 mK. The value of the d.c. susceptibility below the transition indicates the existence of a spontaneous magnetization and so of a ferromagnetic component. The transition remains first-order up to about 200 Oe. In larger fields the magnetization anomaly softens and shifts to higher temperatures.

C. Paulsen is warmly acknowledged for the use of his magnetometers. J. Debray is acknowledged for cutting and orienting the crystal. E.L. thanks S. Petit and B. Canals for fruitful discussions. S.R.G. greatfully acknowledges the support of the European Community Ñ Research Infrastructures under the FP7 Capacities Specific Programme, MICROKELVIN project number 228464.

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