1998ApJ...507..173F

In this paper, historical optical (UBVRI) data and newly observed data from the Yunnan Observatory of China (over ∼100 yr) are presented for BL Lacertae. Large variations of ΔU = 5.12 mag, ΔB = 5.31 mag, ΔV = 4.73 mag, ΔR = 2.59 mag, and ΔI = 2.54 mag, and color indices of U-B = -0.11±0.20, B-V = 1.0±0.11, V-R = 0.73±0.19, V-I = 1.42±0.25, R-I = 0.82±0.11, and B-I = 2.44 ± 0.29 have been obtained from the literature. The Jurkevich method is used to investigate the existence of periods in the B-band light curve, and a long-term period of 14 yr is found. The 0.6 and 0.88 yr periods reported by Webb et al. are confirmed. In addition, a close relation between B-I and B is found, suggesting that the spectrum flattens when the source brightens.