TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4201 SUBJECT: GRB 051105A: TNG optical observations DATE: 05/11/06 01:35:56 GMT FROM: Stefano Covino at Brera Astronomical Observatory S. Piranomonte (INAF/OAR), S. Covino (INAF/OAB), A. Antonelli (INAF/OAR), D. Malesani (SISSA), T. Mineo (INAF/IASFPA), G. Tagliaferri (INAF/OAB), A. Magazzu' (INAF/TNG), N. Pinilla-Alonso (INAF/TNG) on behalf of the CIBO collaboration report: We observed the field of GRB 051105A (Mineo et al., GCN 4188; Cummings et al., GCN 4190) with the TNG equipped with DOLORES on October 05 from 19:41 UT to 20:41 UT (more than 13 hours after the burst) in the R band. Preliminary analysis shows that inside the XRT SR1 (Mineo et al., GCN 4195) error circle one only object at coordinates (J2000) RA, DEC = 17:41:09.95, 34:55:44.5 (error +/- 1 arcsec) is visible. The object magnitude is R=21.19 +/- 0.03 assuming R=17.0 for the star U1200_08571196 at coordinates RA,DEC = 17:41:14.05, 34:55:35.1. This object is also present in the USNO catalogue. One more fainter source is located just outside the error circle at coordinates RA: 17:41:10.25 and DEC: 34:55:39.9. These two sources are probably the same detected by Klose et al. (GCN 4196). Moreover, a very faint source, close to our detection limit, might also be present at the position of the radio source reported by Frail et al. (GCN 4199). We acknowledge the support received from the TNG staff. Further observations are planned. This message may be cited.