OPTICAL SPECTROSCOPY AND POLARIZATION OF A NEW SAMPLE OF OPTICALLY BRIGHT FLAT RADIO SPECTRUM SOURCES

A new sample of bright flat radio spectrum sources selected at 8.4 GHz and consisting of objects brighter than V=17 is discussed. The sample was selected with three purposes in mind: (i) to find low-luminosity BL Lacertae (BL Lac) objects with radio luminosities comparable to tho se of BL Lacs selected at X-ray frequencies; (ii) to investigate the d ifferences between BL Lacs and other flat radio spectrum sources; and (iii) to define a sample of nearby radio-loud objects, the host galaxi es of which are easy to study. Using information on four observational parameters, radio polarization, optical percentage polarization, brea k contrast and equivalent width of the strongest emission line, we com pare the properties of BL Lacs with those of other types of active gal actic nuclei (AGN) found in the sample, We find that most of the objec ts have weak emission lines although some sources with Seyfert-type sp ectra were also found, With only a few exceptions, the two types of so urces appear well separated in their observational properties. Among t he objects studied we report 10 new BL Lacs and BL Lac candidates, and we define a 'complete' sample of bright flat radio spectrum sources t hat consists of those objects with redshift less than or equal to 0.1.