TITLE: GCN CIRCULAR NUMBER: 6651 SUBJECT: GRB 070721B: optical observations and proposed redshift DATE: 07/07/22 19:37:46 GMT FROM: Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center D. Malesani (DARK), P. Jakobsson (Univ. Hertfordshire), J.P.U. Fynbo, J. Hjorth and P.M. Vreeswijk (DARK) report: We observed the optical counterpart of GRB 070721B (Ziaeepour et al., GCN 6640; Schady, GCN 6641) with the ESO-VLT equipped with FORS2. Close to the UVOT position (Schady, GCN 6641), we detect two objects: 1: RA = 02:12:32.97, Dec = -02:11:41.0 2: RA = 02:12:33.03, Dec = -02:11:41.6 We note that object #1 is consistent with the UVOT position, while #2 is 1.6" away. We estimate R~23.8 and R~24.3 on Jul 22.319 UT (21.6 hr after the GRB) for objects #1 and #2, respectively. We acquired spectra for both objects. Thanks to the detection of a DLA and several metallic lines, we infer for object #2 a redshift z=3.626. The spectrum is typical of a GRB afterglow, and we thus suggest that object #2 is the optical counterpart of GRB 070721B. This redshift is also consistent with the nondetection of the object in filters bluer than V (De Pasquale & Ziaeepour, GCN 6650). Further observations are planned to test the variability of both objects. A finding chart and a plot of the 2D spectrum is available at: http://www.astro.ku.dk/~malesani/GRB/070721B We acknowledge excellent support from the ESO staff, in particular Angela Cortes, Dominique Naef and Yuri Beletsky. We thank Massimiliano De Pasquale for useful discussion.