Redshift limits of BL Lacertae objects from optical spectroscopy

Context. BL Lacertae objects have been the targets for numerous recent multiwavelength campaigns, continuum spectral variability studies, and theoretical spectral and variability modeling. A meaningful interpretation of the results of such studies requires a reliable knowledge of the objects' redshifts; however, the redshifts for many are still unknown or uncertain. Aims. Therefore, we hope to determine or constrain the redshifts of six BL Lac objects with unknown or poorly known redshifts. Methods. Observations were made of these objects with the MDM 2.4 m Hiltner telescope. Although no spectral features were detected, and thus no redshifts could be measured, lower redshift limits were assigned to the objects based on the expected equivalent widths of absorption features in their host galaxies. Redshifts were also estimated for some objects by assuming the host galaxies are standard candles and using host galaxy apparent magnitudes taken from the literature. Results. The commonly used redshift of z=0.102 for 1219+285 is almost certainly wrong, while the redshifts of the other objects studied remain undetermined.