MILLIMETER OBSERVATIONS OF X-RAY-SELECTED BL LACS

I present the first 1.3-mm observations of a complete sample of 10 X-r ay-selected BL Lacertae objects (XBLs), together with complementary me asurements of a complete sample of nine radio-selected BL Lacs (RBLs). Three XBLs have firm detections and one has a marginally significant detection. None of the XBLs has a radio spectrum rising steeply into t he millimetre region, and the radio-millimetre spectral indices of the XBLs detected are within the range found for the RBLs. However, the o verall millimetre-X-ray spectra of the XBLs are much flatter than for any RBL.