TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1502 SUBJECT: GRB 020813: addendum to GCN 1498 DATE: 02/08/17 11:30:50 GMT FROM: Stefano Covino at Brera Astronomical Observatory S. Covino, D. Malesani, G. Ghisellini, P. Saracco, G. Tagliaferri, F. Zerbi (Observ. of Brera, Milan, Italy); S. Di Serego, A. Cimatti, M. Della Valle (Observ. of Arcetri, Florence, Italy); F. Fiore, G.L. Israel, L. Stella (Observ. of Monte Porzio, Rome, Italy); M. Vietri (Univ. Rome 3, Italy); N. Kawai (Tokyo Tech, Japan); D. Lazzati (IoA, Cambridge, UK); S. Ortolani (Univ. of Padua, Italy); L. Pasquini (ESO, Germany); G. Ricker (MIT, USA); E. Le Floch, P. Goldoni, F. Mirabel (CEA, France) report: In GCN 1498 we claimed that the polarization measurements performed by Barth et al. (GCN 1477) and our group on the GRB 020813 afterglow allow to single out "the most significant and larger degree of variability ever detected in the polarization level of a GRB optical afterglow". However, according to the result of Bersier et al. (astro-ph/0206465) a large amount of variability in the polarization level could have been present in GRB 020405 if compared to the polarization level measured again by our own group (Covino et al., GCN 1431). In this case, the measurements performed for GRB 020813 are only a confirmation for the large variability in a short time scale (~24 hours) in the polarization level of a GRB optical afterglow. This message may be cited.