TITLE: GCN CIRCULAR NUMBER: 17738 SUBJECT: GRB 150423A: VLT detection DATE: 15/04/23 20:20:49 GMT FROM: Alexander Kann at TLS Tautenburg D. A. Kann (TLS Tautenburg), T. Kruehler (ESO Chile), S. Klose (TLS Tautenburg) and J. Greiner (MPE Garching) report: We obtained a photometry sequence in BVRIz with VLT FORS2 of the optical afterglow (Varela et al., GCN # 17729, # 17732; Perley, GCN # 17733; Littlejohns et al., GCN # 17736) of the short GRB 150423A (Swift-638808, Pagani et al., GCN # 17728). Observations started on April 23, 2015, 08:02:19 UT, 94 minutes after the GRB. Seeing was 1".0 and airmass was 1.7. The afterglow is clearly detected in BVR. Analysis of Iz is pending. We find Rc = 22.8 +/- 0.1 in the first image (calibrated against SDSS standard stars transformed to Rc) and find it fades by approximately 0.4 magnitudes in the course of an hour (see also Perley, GCN # 17733; Littlejohns et al., GCN # 17736). We are indebted to Jonathan Smoker and Fernando Selman for VLT UT1 support.