The stability of the frequency content in the lightcurves of the delta Scuti stars HD116439 V663 Cassiopeiae, AZ Canis Minoris, HD 223480 BF Phoenicis

Within the frame of the study of the pulsational behaviour of delta Sct stars, multicolour photometry of (previously) considered monoperiodic or double mode pulsators was undertaken. The frequency content of the light variation of SAO 4710=HD 16439=V663 Cas is clearly changing as a new frequency, not present in 1989 was observed in 1995; a slight decrease is also observed in the amplitude of the dominant term. The light curve of V663 Cas is now explained by three frequencies, but the residual noise probably masks other terms. SAO 231800=HD 223480=BF Phe is confirmed to be a monoperiodic delta Sct star, but comparing the 1989 and 1993 photometric measurements a strong reduction in the b amplitude is observed; on the other hand, the V amplitude remained stable in 1991 and 1993. The monoperiodic light curve of HR 2989=AZ CMi seems more stable, giving only slight hints that small changes occurred. The different photometric behaviours of these stars are discussed, also by comparing them with other monoperiodic pulsators (beta Cas, 28 And, tau Peg).