Long-term Variation of AGNs
  In this paper, we will discuss the long-term variations in the optical and the infrared bands of some AGNs. It is interesting to note that the reported periods of AGNs are of the similar values (about 10 years, see Fan et al. 1998a, ApJ, 507 in press, and references therein). DCF method shows the optical and infrared bands are strongly correlated suggesting that the emission mechanisms in the two bands have a common origin. 
J.H. Fan, G.Z. Xie, G. Adam, S.L. Wen, Y. Copin, R.G. Lin, J.M. Bai,   and Y.P. Qin