GRB 221009A: OHP optical observations

We observed the field of GRB 221009A (Dichiara et al., GCN 32632; Veres et al., GCN 32636) using the T120cm telescope at Observatoire de Haute-Provence (France). Four exposures were obtained in the r band (2x300s + 2x600s), three exposures in the i band (3x600s) and the z band (1x300s + 2x900s) from 2022 12 Oct 19:11:05 UT to 2022 12 Oct 20:41:23 UT (mid time ~78h after trigger). In the combined frame, we clearly detect the source in the three filters and obtain the following magnitudes: r = 20.23 ± 0.09 mag (AB) i = 18.91 ± 0.11 mag (AB) z = 18.35 ± 0.13 mag (AB) The photometric calibration was performed using nearby stars from the PanSTARRS catalog and magnitudes are not corrected for Galactic extinction. These values are consistent with observations obtained at a similar time by Bikmaev et al. (GCN 32743). Further observations using the MISTRAL instrument mounted on the T193-OHP are planned for the coming nights. We acknowledge the excellent support from Observatoire de Haute Provence, in particular François Huppert and Jérome Schmitt.