GCN Circular 21737
P.G. Jonker (SRON/RU), M. Fraser (UCD), S. Nissanke (RU), R.P. Fender (Oxford), J. Broderick (ASTRON), A. Rowlinson (UvA, ASTRON), R.A.M.J. Wijers (UvA), B.W. Stappers (Manchester) report on behalf of the LOFAR Transients Key Science project: On Aug, 27, 2017, starting at 03.37 (UTC), we obtained three spectra with an exposure time of 900 sec each, with the ACAM instrument mounted on the William Herschel Telescope on La Palma (Spain) of the optical transient MASTER OTJ033744.97+723159.0 reported by Lipunov et al. (GCN 21720). Analysis using Gelato suggests that it is a supernova Type IIb before maximum taking the redshift of NGC1343 of 0.0073 as input.