Multicolour ( UBVRIJHKL ) photometry is presented for the two peculiar objects AP Lib ( = PKS 1514–22), which is thought to resemble BL Lac, and PKS 0521–36, an N-type galaxy. Both objects have spectra typical of synchrotron emission and are similar in becoming redder as they get fainter, in the manner of BL Lac.
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Andrews et al. (1974) studied this question.