Longitudinal magnetic field variation of the Of?p star HD 148937 according to the 7.032 d period determined by Nazé et al. (2010). Red symbols correspond to the ESPaDOnS observations (Wade et al. 2012b), while green symbols are our FORS 1 and FORS 2 measurements (Hubrig et al. 2008; 2011b and present paper). Note that the measurement errors for both ESPaDOnS and FORS 1/2 observations are of similar order. One more measurement (not shown in this figure) was obtained in 2010 on May 22, but ordinary and extraordinary beams in the FORS 2 setup were overlapping in this exposure due to problems with slitlets.