Historic light curve and long-term optical variation of BL Lacertae 2200+420

In this paper, historical optical (UBVRI) data and newly observed data from the Yunnan Observatory of China (over similar to 100 yr) are presented for BL Lacertae. Large variations of Delta U = 5.12 mag, Delta B = 5.31 mag, Delta V = 4.73 mag, Delta R = 2.59 mag, and Delta I = 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.