TITLE: GCN CIRCULAR NUMBER: 23040 SUBJECT: GRB 180720B: OAJ optical observations DATE: 18/07/26 08:56:58 GMT FROM: Luca Izzo at IAA-CSIC L. Izzo, D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), C. C. Thoene, K. Bensch, M. Blazek (HETH/IAA-CSIC), M. C. Diaz-Martin, and S. Rodriguez-Llano (OAJ) report: We observed the field of the Swift-BAT GRB 180720B (Siegel et al. GCN 22973), detected also by Fermi-LAT (Bissaldi et al. GCN 22980), with the 0.8m telescope of the Observatorio Astrofisico de Javalambre (Teruel, Spain). Observations consisted of a series of 3x300 s griz exposures, starting at 01:12:55 UT (10.85 hr after the GRB trigger). The afterglow is clearly detected at a position consistent with the one reported by Martone et al. (GCN 22976). We measure a magnitude of r(AB) = 17.77+/- 0.05 mag at an average time of 01:37:14 UT (11.26 hr after the GRB trigger), as compared to nearby SDSS stars.