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The large majority of BL Lacertae objects belonging to the 1 Jy sample, theclass prototype for radio-selected sources, are thought to emit most of theirsynchrotron power in the far IR band. Ironically, this spectral region is verysparsely sampled, with only a minority of the objects having IRAS data (most ofthem being upper limits or low-quality detections). We aim at filling this IRgap by presenting new, simultaneous ISOCAM and ISOPHOT observations over the 7- 200 micron range for half the sample. A measurement of the position of thesynchrotron peak frequency, nu_peak, can provide information about particleacceleration mechanisms and constrain the inverse Compton radiation that willbe detected by up-coming new gamma-ray missions. We have observed 17 1 Jy BLLacertae objects with the camera and the photometer on board the Infrared SpaceObservatory (ISO) satellite. The ISOPHOT data reduction was done employing anovel correction, which mitigates the effect of chopping for faint sources.Using our new ISO data, complemented by nearly-simultaneous radio and opticalobservations for 10 and 4 objects respectively, and other multi-frequency data,we have built the spectral energy distributions of our sources (plus apreviously published one) and derived the rest-frame nu_peak. Its distributionis centered at 10^13 Hz (30 micron) and is very narrow, with 60% of the BL Lacsin the 1 - 3 10^13 Hz range. Given our set of simultaneous infrared data, theserepresent the best determinations available of the synchrotron peak frequenciesfor low-energy peaked BL Lacs. A comparison with previous such estimates, basedon non-simultaneous optical and near IR data, may indicate strong nu_peakvariations in a number of sources, possibly associated with large flares asobserved in the high-energy peaked BL Lac MKN 501. (abridged)Comment: 13 pages, 1 figure, accepted for publication in Astronomy &  Astrophysic

Filling the infrared gap: ISO observations of 1 Jy BL Lacertae objects