Abstract

We present optical photometry (⁠|$B, V, R$|⁠, and |$I$|⁠) of the BL Lac object ON231 (1219|$+$|285, |$z$||$=$|0.102), which was observed with the 1 m telescope at Yunnan Astronomical Observatory and the 1.56 m telescope at Shanghai Astronomical Observatory from 2003 May to 2006 May. Our results indicate that (i) there is a short time variability of |${\Delta} I =$| 1.04 mag during 25 minutes; (ii) there is a strong correlation between the |$R-I$| color and the |$R$| magnitude in 72.2 hours; and (iii) our optical variability is consistent with those of Tosti et al. (2002, A&A, 395, 11).

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