Abstract
We present the late-time optical photometry of supernova (SN) 1993J in M81 from 1995 February to 2003 January. The observations were performed in a set of intermediate-band filters that have the advantage of tracing the strength variations of some spectral features. SN 1993J was found to fade very slowly at late times, declining only by 0.05 ± 0.02 mag (100 days)-1 in most of the filters from 2 to nearly 10 yr after discovery. Our data suggest that the circumstellar interaction provides most of the energy that powers the late-time optical emission of SN 1993J. This is manifested by several flux peaks seen in the rough spectral energy distributions constructed from the multicolor light curves. The flux peaks near 6600, 5800, and 4900 Å may correspond to the emission lines of Hα, Na 
D + He λ5876, and [O III] λλ4959, 5007, respectively. The evolution of these emission lines suggest a power-law SN density model, as proposed by Chevalier & Fransson.