We present simultaneous multifrequency radio observations for a complete
subsample of 26 XBLs from the Einstein Extended Medium-Sensitivity Survey,
obtained with the Very Large Array (VLA). Spectra are computed using fluxes at
20, 6 and 3.6 cm. Unlike many radio selected samples, the EMSS did not impose
any criterion on the radio spectrum to identify BL Lac objects. It is therefore
possible to investigate the intrinsic radio spectral slope distribution and to
determine the effect produced by this selection criterion. We find that 15% of
the observed objects do not meet the flat-spectrum criterion imposed on some
other BL Lac samples. A dataset that includes non-simultaneous data (that are
also taken with different VLA configurations) shows an even higher percentage
of steep spectrum sources. This effect can be ascribed to a larger fraction of
extended flux detected with the more compact VLA configuration.Possible biases
introduced by the flat--radio-spectrum criterion in the radio-selected BL Lac
samples cannot explain the discrepancies observed in the evolutionary
properties of Radio and X-ray selected samples of BL Lacs.Comment: 9 pages, 5 figures, to be published in Astronomy and Astrophysic
