2000A&A...361...60S

We present Very Large Array (VLA) 5 GHz continuum observations of six point sources towards the halo of M 31, which have featureless optical spectra and magnitudes ranging from V=21.5-22.2, in order to determine whether these objects are BL Lacs. No radio emission coincident with the optical positions is detected to a 5σ noise level of between 0.08 and 0.11mJy. The resulting upper limits to the two-point radio-to-optical spectral indexes from 5GHz to 2500Å (α_RO) are less than 0.30 for all of the sample. These observations make it highly likely that these objects are not BL Lacs, and hence strengthens the case that they are normal B-type stars in the halo of M 31.